Skeletons sometimes hold an invisible bow

Was the mob a natural spawn or did you spawn it in with commands?

Your description is considered "Working as Intended" (per MCPE-23165) but should only be possible when the mob is spawned in with commands. Its unclear if your summary implies it was natural.

Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.

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Still affects 1.16.210 it also affects piglins too, it charges a invisible crossbow, and works with other mobs too

Similar to others like MCPE-120071

Natural spawn