When skeletons are spawned with a melee weapon (so any item except for a bow), they have troubles with attacking and swing their arms rapidly.
Sorry if this is a duplicate, I tried searching but couldn't find anything.

This applies to zombies too apparently; they sometimes cant hit you even though they are standing right in front of you.

EDIT: Somewhat related to https://bugs.mojang.com/browse/MC-87093 (arm swinging issue)