execute as @e[type=skeleton] run data merge entity @s {NBT in skeleton}
and write it down in a repeating command and always activate it.
MC-264509
If you keep changing the skeleton's NBT, the skeleton will not be able to attack.
Resolved
Won't Fix
0
Confirmed, relates to MC-165448 and MC-151468 (maybe should be a duplicate of the latter, and generalizing it).
This can quite easily be avoided by map makers though by only attempting to modify the data when the data doesn't already match (execute unless data entity @s {...} run data merge entity @s {...})