I’ve seen this happen a few times recently (a few months maybe).
The last boss, when he starts doing his spin attack, if cc-ed (too fast?) will sometimes start the spin charge up attack immediately after his stun wears off.
If cc-ed again, he will attempt his spin attack immediately after that 2nd stun, and he will do this until the group runs out of cc and he finally activates his spin.
Basically if the bug triggers the only way to get him out of his permanent and instant charge up spin attack is to let him do it, at which point he will return to the normal wait period between spins.