[Solved] Inconsistent Water Temperature After Replacing Thermostat Faucet with Regular One

Hello, some time ago I replaced the faucet with the rain shower with an old type, i.e. without a thermostat. Everything was fine until the day before yesterday. When I turn on the hot and cold water, lukewarm water comes out, and then it changes itself, sometimes to cold, and sometimes to warm. This cannot be adjusted. What could be the reason? I previously had a thermostat faucet, but it kept breaking down. From what the plumber said, it was clogged due to hard water - scale was accumulating and he recommended that I replace the bacteria with a regular one. Please help me what can be done about it because I can`t imagine replacing the battery every three months on average.

Or maybe it`s an apartment in a block of flats and there are jumps in the pressure of hot and cold water when someone turns on/off the tap on the same riser, and then the water with the higher pressure comes out.

Describe what house/apartment, what installation, whether with an additional circulation circuit and pump, what hot water tank and how it is connected, what water temperature can flow after turning on the hot water? If in a block of flats, which floor is good, and what is the water pressure, for example?

The water pressure is good and I have no problems with it. This didn`t happen before. Apart from that, this is the only problem in the shower tray, everywhere is fine. I don`t know what the connections are exactly because it`s an apartment in old blocks from the 1960s. How can I check it?

I replaced the battery and the grandfather. It just broke.

I replaced the battery and it works. It just broke.

Added after 44 [seconds]:

I replaced the battery because it was broken.