Unkillable Mobs by accident

It does a lot of time i have this bug and... maybe one of you can find what's going on. I have a tower defense where Mobs are NoAI, resistant, persistent, etc... everytime a tower hurt them, it removes scores (not health). When the score hits 0 or less, the mob is killed by the system. Sooometimes, at random rates, some of theses killed mobs will apparently stay. They get a kind of invulnerability : we can't hurt them (even in creative OP), we can't kill them (it announces the kill's done in the chat but this is not what happens) and the mob stay forever until the reset of the game set the difficulty to peaceful... I got the data of the mob : no invulnerability, no effets... normal Health. How do my game's creating some REAL dead-alive mobs ?! Others infos : - the skeleton, stray and skeleton_horse on the screens are dead-alive too - the commands don't find them (they stop going forward, like they are dead) but the others normal mobs do it, si the system is working fine (modifié) - but we can data get entity them, kill them manually verify their tags, etc... If i active the A.I., they'll rotate facing the player like a normal mob but they don't moove anymore - this is the ONLY game on 72 gamemodes on my server where this bug happens