The title pretty much says it all; After the introduction of the ability to take enough damage in a vehicle to reach the 'knocked out' state with the Vanilla Medical system and not die instantly, this issue was presumably introduced in a bugfix where AI hostiles no longer shoot downed players.
Basically, if you (the player) turn out the top of a tank in the Commander seat and get knocked out but not killed, a player in Spectator (how I found out) can notice your tank is now marked as purple.
I'm not entirely certain if this actually affects the AI's ability to engage the vehicle or if the destruction of the vehicle counts towards a Player's Renegade stat, but it could and that's worth a bug report.