The bug

All variants of skeletons can hold their bows with one hand when spawned with a bow in their offhand which resembles skeletons before 1.9, but still has the same AI as of today.

How to reproduce

1.  /summon skeleton ~ ~ ~ {HandItems:[{},{id:bow,Count:1b}]} 

   This does not only affect skeletons however. Withered skeletons and stray skeletons are also affected.

2. Go to survival
   → The hostile skeleton holds its bow with one hand, which resembles the old skeleton AI before 1.9, but the bow retains the pulling animation.