Ok, now I know why. without the +1 the automation does not trigger when the temperature is over the target and the TRVs running on idle. The +1 triggers the automation and switches them off until the room temperature is under the target temperature. So it does not explain why the mode is switched from heat to off or the other way around in the target +1° range.
I use the generic thermostat but I want to switch the TRVs to different temperatures/states depending on a schedule and if we are at home or not. The automation changed over the last 3 or 4 years but basicly it was one of my first experiments with HA. It worked flawless and did what it should.
They are Sonoff TRVZB. Bosch changed the firmware and with 3.7.4 the TRVs startet to recalibrate and reboot at random times. Sometimes 15 minutes in a row. Bosch said: Thats your valves, change them, not our fault.
Newest version is working for me. So, let's see, what the future brings.