Treat your skin gently

When shower faucet apart make sure hot is getting 2 & coming out faucet ***. did you turn off stop integral w/ faucet or outgoing shut off valve on water heater.. May have rust or lime in small orifice leading to faucet ***. does your balancer/ piston move freely..good luck..

Try running the hot water in the sink while you have the shower. If the water in the shower stays hot, then the problem is not enough flow through the water heater to keep the heater running. I am guessing that the master bath is further away from the water heater than the guest bath. If not I would check the shower nozzle in the master bath to see if it is clogged a bit or if it is a water saver nozzle because this will slow down the water volume and then cause the water heater to shut off the heat.

Paragon 8145-20 is a 240V defrost timer
It is probably not best choice for water heater timer
Timer initiates defrost cycles each 4 hours.

A water heater timer should have more scheduling options.
-For example estimates show average run-time for water heater is 3 hours per day, depending on incoming water temperature, thermostat setting, and number of gallons used.
-To save money using water heater timer, you have to schedule times carefully so water heater runs less than 3 hours per day.
-Now your average electric water heater heats about 20 gallons per hour.
-Average shower uses 4-9 gallons hot water, so water heater needs to run 15-30 minutes (15 summer, 30 winter)
-Average bath uses 12-15 gallons hot water so water heater runs 45-60 minutes
-But lets say you run water heater for 1 hour and make 20 gallons of hot water. Then you take a shower, expecting hot water to remain in tank.
-Here's the problem ... cold water entered the tank to replace the shower water ... and the new cold water is cooling the rest of the tank.
-So a timer means scheduling hot water to match shower and bath events. For example taking shower and bath in immediate succession.
-Or set the timer for each individual hot water event ... for example set timer for 1/2 hour for shower, and later timer turns on 1 hour for bath, or 1-1/2 hr for 2 baths, etc
-> The flexible programming described above is not a characteristic of defrost timers.
I suggest Intermatic WH-31 240V or Intermatic T-104 240V or GE 15207. These are mechanical repeat-24 hour timers with manual override where each day has same schedule.
Digital programmable timers like Intermatic EH40 offer 7-day, 40 event programming. Weekends and weekdays can be different.
To see several box-type timers:
http://waterheatertimer.org/Boxtype.html

its called ur anti-scold system its installed in all single shower units to adjust remove the handle and will see usually a blue and red adjustment clips move the red so more hot water can be mixed in try the water adjust until comfort level is achieved

Try draining the system and apply air pressure (min 30psi) to each faucet. Open other hot water faucets to allow pressure to vent. Next try draining and cleaning the hot water tank.