I don’t think this is related to HomeKit.
Afaik the central units are not fully supported.
Pls read here:
And the binding documentation:
Thermo zones management and temperature sensors (probes). NOTE Central Units (4 or 99 zones) are not fully supported yet. See Channels - Thermo for more details.
For my understanding you need to change the setpoint in the central unit too (not only at the thermostat) or it will be overwritten with the old value from the central unit.