As OP has said, it should be up to players to remember their own lockouts. At the very least, might I suggest adding a tip to the number you have reminding players to use the correct subtype for their raids?
Better yet, if you wish to keep the system in place, make it so when you attempt to waitlist for a group with a lockout conflict, bring up a warning screen telling the player of the conflict, but allow them to ignore it and waitlist for the group anyway. This way, if it is indeed a bug at the time, it won't stop the player from raiding.