Thermostat/boiler set to opentherm yet still turning off boiler?

Hello, I have a Thermomaster 3HR boiler coupled with 1 wired Tado thermostat and 10 TRV's. The radiator valve in the bathroom is a manual valve, always open on full.

I used to have the system set to on/off, but now I'd like to switch to the OpenTherm modulating setup. So what I did was change the cables from the thermostat from NO/COM to + and -, and change the cables on the boiler from on/off to opentherm. If I understand correctly it doesn't matter which cable goes where because opentherm doesn't care about this.

The heating system works, but the boiler still turns off when the room has reached its desired temperature. Why could this be?