Climbing to cruise and centre started telling me QNH and instructing me to descend >>>

Without the flight plan and log file, I'm just guessing.  But it sounds like your route is fairly short or your cruise altitude is too high for the length of the route and you reached the TOD point before you got to cruise altitude.  The TOD that ATC uses may be different from the one displayed on the map because it will be modified for things like planned approach, vectoring, STAR assignments, etc.

Dave