@Wayne-TP Yes, my TRV is linked (bound) to a KE110 smart thermostat. The TRV eventually updated it's firmware to v2.10.0 (but not the v2.4.0 mentioned in other posts that it apparently needs)

When tapping the Boost button, a timer window briefly appears with 00:00 set and a temperature of 30C, but this immediately disappears and the temperature reverts back to is original value. Manually changing the temperature also doesn't work, it just reverts back to is original temperature after a few seconds (I have tried different Hold Durations).

I think the whole thing is broken and am tempted just return it and try a different  brand - it is not even following the smart schedule settings.