Water – Hot water tank only has warm water after bath

If they've already checked everything outside your unit, then the next thing to check is the mixing valve in your shower. Especially since your sink water gets hot enough.

New Answer Derived From Exhausting Initial Answer & more:

It's your Water Heater, here are some things to try in determining whether or not there's actually a problem.

1 - Drain & clean the tank properly. ONLY do it when the pressure's good. Turn off the Hot & Cold Water & WH, open the T&P valve & drain the tank (see manual) to somewhere that contents can be seen. Blast on the cold water when you can start to "slightly" rock the tank to stir & break things up & periodically continue this until it's empty. Operate the hose bib handle to crush any big particles or Poke into your fully open hose bib if the hose flow drops. Then, when empty blast on the cold water & let drain & repeat until nothing comes out. Then, do just a 2-minute fill & drain & blast again to get all remnants out.

2- Check your status lights to your manual for detected problems.

3- Status Lights will also indicate a problem with the heater's Gas Control valve. But, this could also be a Gas Meter response to a run-away or leak detector, improbable since flow-rate wouldn't have changed.

4 - This is a pretty cutting edge unit you have & everything's interconnected. The fan & burner won't outrun each other & there may even be a Water Exit Regulator involved to turn the heater, sort of, into a Tankless Water Heater.

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Original / Initial Wrong Answer: If you have a Hot Water Recirculating Pump somewhere else, you're driving its thermostat batty & it's only now showing signs of needing attention. It would be stealing pressure until it's reading hot water again. You may have noticed the pressure returns after 10 to 30-minutes, when the Water Heater has recovered enough or recovered fully.