Whenever I start the engine, I tap ‘Log Engine Start’. When I start to taxi (from hangar to apron) SD automatically starts counting the off block time. When I stop on the apron and let the engine warm up, the off block time counter stops counting, restarting to count when I taxi to the holding point.

My observation is that the block time-counter stops counting while you are not moving on the ground and resumes when you start moving again. The counter however seems to continue running in the background even while you are stopped. So in the end it displays the correct block time as defined by FCL.