Skeletons spawn with no AI, can be hit once, and then will not take more damage.

It also happened in a previous w43 snapshot (haven't tested in 43c) where a spider spawner rapidly spawned these AI-less skeletons that could not be killed, and I think they eventually fill up the server's RAM and crash it as they cannot be killed and I don't think they despawn. They can only be killed using kill @e[Type=Skeleton].
They will take one hit, flinch, and then not take any more damage with subsequent attacks.
They don't move.
They don't attack.
They may or may not spawn with armor.

This isn't to say all skeletons are broken, some work as intended, but many do not.