Skeleton isn't burning in sunlight, floating and can only hit once.

One skeleton didn't burn in the sunlight for whatever reason. I tried removing the block beneath it ,causing it to float in midair, and then tried to hit it. I hit it once and all expected effects occured (flashed red, particle effects etc.). It is unhittable after this, meaning I cannot physicially touch it. See screenshot below. ^note It's idle animation is running but without any sound. ^note