It is a direct quote from the A320 FCOM (probably similar for other Airbus types). DSC-22-30-90 A/T Arming Conditions.
During take off the A/T is armed but not active (FLX and TOGA are not A/T modes), if you set it to idle (aborted take off for example) it disengages and can only be rearmed once airborne (or after the aircraft has been shut down and started up again). FLX and TOGA continue to work though. At least that is my understanding of the note, never tried it.
By the way, a question like this is probably best posted in Tech Log.