This bug is related to mimikyu's broken disguise not resetting its typing (noticed in camomons - either this is a visual glitch, such that camomons doesn't show mimikyu's camomon typing after it is busted, or an actual bug that affects all tiers, such that mimikyu's typing shows as ghost-fairy, but it retains the changed typing. Testing is necessary.) Disguise, when busted, should reset mimikyu to its original typing. But in camomons, a normal-ghost mimikyu, after its disguise was busted and it was no longer labeled as a normal-ghost type, was still immune to ghost moves.
Bulbapedia mentions that " If Mimikyu's type was changed, it is restored to
Ghost/
Fairy when it changes to its Busted Form". This wording can be interpreted as "always changes to ghost/fairy" (unlikely) or "changes back to original typing" (more likely). Either way, a bug exists such that a mimikyu
that was no longer labeled normal/ghost was still immune to ghost moves after the disguise was broken.