When executing "/summon Skeleton ~ ~ ~", the Skeleton has his bow.
But when one or more NBT Tag is specified and HandsItem is not one of them (example: "/summon Skeleton ~ ~ ~ {LeftHanded:1}"), the Skeleton spawns without his bow. The same happens to Wither Skeletons.
Linked issues
is duplicated by 16
MC-102185
If the "SkeletonType" or "Type" tags are used w/ /summon, the mobs get no weapons
Resolved
MC-103202
skeletons variants not spawns with equipment (Sword and bow)
Resolved
MC-108453
Vindicators' axes are invisible when inside a team
Resolved
MC-110158
NoAI tag causes mobs with default handheld items to lose them
Resolved
MC-125782
Phantom Initial AI movement with NBT data
Resolved
11 more links
If you specify a NBT tag, it won't automatically add the default equipment to mobs. In this case you have to specify it in the NBT