When a Skeleton/Wither Skeleton attacks a target at melee, it will sometimes get stuck doing a bugged out animation where it quickly raises and lowers its Melee weapon (If it has one), and will only sometimes do damage this way. They also will sometimes go for a couple seconds where they simply ignore their target, even if they are damaging the skeleton. This is possibly two glitches, as there appears to be both a problem with pathfinding and the arm swinging glitch. 

This only has an effect on 1.14 and 1.14.1, as it was NOT an issue in 1.13 (I tested it)