For baseboard V1-0, when using Homekit, the inactive brightness is automatically set to half of the active brightness. Therefore, if the brightness is set to 10%, half of it is 5%. The thermostat can only change the brightness in 10% increments. So, 5% would turn off the thermostats.
If you find yourself having any other brightness issues, try turning off the circuit breaker and turning it back on again.
To figure out how to adjust the brightness, click here.