Shower water temp/pressure issue in tiny house- totally stumped

I recently finished a tiny house build (my dad and I did most of the work, including plumbing, ourselves) and moved into it in August. There was a small issue with the water temp- namely, that I couldn't regulate the water temp in the shower or kitchen. I worked around it by adjusting the temp on my small tankless on demand heater, and the water that came out was the temp I set that at. No problems, just a bit of an annoyance.

Fast forward three months, I'm now living out of state and have a tenant living there. About two weeks ago, the water temp started dropping in the shower (just the shower, not the kitchen!), and is now settled at "lukewarm". When she tries to turn the shower knob to what would be hot, the pressure drops.

We tried doing a valve flush to get rid of any debris, but it didn't solve the problem and we're all totally stumped as to why a brand new build like this would develop this issue after working just fine for two months. Any ideas would be so appreciated!

The small issue is that your original plumbing design must have been wrong.

Or you installed two faulty faucets.

Or plumbed them wrong.

Time to hire a plumber, you are out of state.
Unless you expect your tenant to fix it for you now.
You would be exposing yourself to big liability by asking your tenant to try to fix it.

You should never have rented it with no direct control of water temp at the fixtures.
Very dumb.

Meanwhile, it sounds like the cartridge in the shower valve has gone bad.

But even if your dad or someone else replaces that, you will just be back at your original poor setup.