Steps to reproduce 

1. /summon skeleton
2. /effect @e[type=skeleton] fire_resistance 10000 (so that it doesn't burn up in the day)
3. /gamemode s
4. Let the skeleton shoot once, then run away before it can shoot again
5. Wait for the skeleton to catch up
   → The skeleton instantly shoots again when it is within range

When skeletons lose sight of a player they keep their arrows charged, but they fire instantly when they see another target