I have several rooms that I don't heat if I'm not using them, so they're programmed at 10° to prevent freezing.
The problem is that when I set the Evohome system to "Away"... it changes these rooms to 16° which is my "minimum temperature".
In my opinion the controller shouldn't set the temperature during "Away" higher than what I request in the schedule (or it should allow me to define a min. temp. per zone)
Is there anyway to avoid this ?
That one is already in use for "Home office".
Nope. I've had this exact discussion with the Honeywell guys on here years ago, (you can probably find the thread with a search) away increasing temperatures (as well as reducing them) to the away set point is "by design" and they don't have any intention of changing it.
I'd suggest just reducing your away temperature. The default is 15C but I've dropped mine to 10C. Low enough that the heating is not going to come on if you're away from home for a few days but high enough that your house is not going to get near freezing or full up with condensation during a prolonged absence.
Personally I feel the default away temperature is quite high - for me the use case for the away mode is that I'm going to be gone most of the day or perhaps 2-3 days on a long weekend. In that case 15 would cause the heating to start running again well within 24 hours whereas 10C wouldn't.