Resolution: Cannot Reproduce
Affects Version/s: 0.16.1, 0.17.0.1, 126.96.36.199, 188.8.131.52, 184.108.40.206, 1.0.0
Fix Version/s: None
Most the flying entities (like fire charges, TNT, wither skulls except ender pearls) don't update when they are out of the player's range or rendering distance. They just freeze then update when the player gets closer.
How to produce an example:
Start in a flat world.
Build the wither in creative.
It should start shooting wither skulls randomly.
Kill it with sword or bow (but not command)
Now start flying away from the wither place (around where it was).
You'll notice the wither heads are still flying close to you (as if they are newly spawned, which they're supposed to be way farther) then they fly away after you get closer to them.