When I flush the toilet, water fills the bowl. After about 30 minutes, the water drains from the bowl completely.

Water is not draining completely from the toilet line to the sewage tank. If there is a 'low/little slope' section anywhere in that drain line, solids WILL settle and collect there and cause water to back-up in the toilet drain line. Then the toilet will not flush completely or at all until the 'soft blockage' is cleared out and the drain line emptied. Al 'gravity drain' toilets require a free draining/empty drain line that allows flush water to drain unrestricted. A basement toilet has a pump that 'forces' flush water up and out with enough pressure to sometimes force out the 'soft clog.'

Sorry but rather than me simply regurgitate what I just read, Google your issue. What I read in one case and it is not uncommon, was a crack in the bowl that was internal and the bowl drained down completely overnight into the drain - No water on the floor where the toilet was located and no water draining through the ceiling below.'
Here's the one link:
https://terrylove.com/forums/index.php?threads/toilet-water-in-bowl-slowly-drains-minutes-after-flushing-has-completed.3066/
Good Luck

Remove and clean the flush valve and the flush valve seat(it is the flap that opens up to allow water to flush the toilet). Once the edges of the valve and the seat are cleaned, possibly with "0000" grit bulldog steel wool, it should not leak anymore. To make sure it is not leaking, once tank has refilled completely, put some food die in the tank. If, after 10-15 minutes, the water in the toilet bowl is colored, you have a leak from the flush valve. Replacing the flush valve is very easy - just bring the old one with you to the hardware/plumbing store and ask for a replacement. Blessings and good luck!!

The bowl is cracked and is leaking down the drain. You will have to replace it. It may have cracked if the bowl hold-down bolts were overtightened, or the bowl was not able to sit flush (pardon the pun) on the floor. If the bowl rocks when it is first installed, instead of tightening the hold-down bolts, use a couple of tapered shims under the edge of it, then caulk to fill the gap and hide the shims.
Good luck.

Sorry for problem with new toilet.
You say water is leaking out of the bowl?

Toilets have 2 parts:
1) Tank: where water is stored for the flush and is sealed with a flapper.
2) Bowl: where water flushes down.

a) If tank is leaking water, then flapper is suspect. Replace flapper and see if that works.
Lift up flapper and run finger around rim to see if there is a rough spot that would indicate flush valve is defective, and needs replacement.
See typical flush valve

b) If bowl is leaking, that is very unusual and would indicate a crack in the porcelain.

If installer sold you the toilet, it is his responsibility to make good on product.
All products are guarantee against workmanship flaws.

Hello. Your toilet is leaking through the flush valve. This is a serious issue as it will use a huge amount of water every day. Check to be sure there is not an obstruction in the tank and if you can not find one, call the plumber. J.

Sounds like you have a clogged drainline or vent-stack. Fill a large bucket with about three gallons of water and dump it quickly into the bowl. If the drainline/vent is clear, the toilet will "flush" like a gravity toilet and the water will drain from the bowl almost as fast as you can pour it in.

If the water level in the bowl slowly rises (as if it's about to overflow) and then slowly drains, you have a clog somewhere.

All toilets, regardless of their flushing mechanism, require a clear, unoibstructed drain/waste/vent system to extract waste.

If the toilet "flushes" with the bucket of water, you may want to call Flushmate tech support (the number is on top of the tank) or check out the installation and troubleshooting videos at howto.flushmate.com.

Hope this helps.

Not being a plumber by trade I don't want to bad mouth what your plumber had to say but there may be other things to consider.
If the toilet does not flush directly into the main drain there is the possibilty that the coupling between the main drain and the toilet drain is under-sized which could cause reduced water flow and your toilet not flushing completely.

First, fyi, adjusting the knob? in the tank, or the fill level will not increase the amount of water in the bowl. The bowl fills to a certain point, and then as more water is added it starts to go over the built in trap, and down the drain. When you flush the toilet, all the water in the tank dumps into the bowl, and the existing water is pushed over the trap and down the drain. All of this being said it sounds like you may either have an internal crack, between the bowl and trap; you may have a crack that is draining under your toilet, (water spots?), or you have a more serious plumbing issue, improper venting, which is causing a vacuum, and pulling the water out. Hope this helps.

The tank holding the proper amount of water is a good sign. The inside of the stool is like the GOOSE NECK under you sink. It always holds water. This may sound like a bit of a hassle to do but if all 3 toilets are identical, here is what you can try if you are a bit handy. Go to the hardware store and purchase 4 toilet wax rings with a plastic flange and a few sets of toilet floor flange bolts(they are cheap and you can always return if not used, as It is better to have xtra parts on hand). Turn off the water supply to the "Mystery Toilet". Leave the tank and everything connected to the stool. Flush the toilet and remove all excess water from the tank and bowl. This can be done with a shop vac, sponge or towel. Disconnect the water supply to the tank. Remove the bolts holding the stool to the floor and carefully remove toilet. Scrap off all the old wax from the floor flange and bottom of stool. Now go and repeat this proceedure to one of the toilets that is working properly. In other words Switch the "Mystery Toilet" with a "Working Toilet" and see how they react. If the working toilet does it in the same bathroom and the "Mystery Toilet" does not do it in the other bathroom then you probably have a VENTING PROBLEM. If the "Mysery Toilet" Does it in the other bathroom then you most likely have a Defective Toilet. This Job is best done by 2 people, as it can sometimes be challenging to center the new wax ring on the toilet flange. Reconnect to the floor and reconnect water supply to the tanks. DO NOT OVERTIGHTEN BOLTS to the floor flange as you may crack the toilet. All drain lines must be vented properly for water to flow. If you are curious check out drain venting on the internet. Also, sometimes a vent can become obstructed with debris, ice, bird nest, etc.. I would 1st check the vent on the roof visually and then have someone flush and listen. If you believe the vent is clear....Then do the toilet switch and go from there.....Hope this helps and Happy Holidays....Joe