Mobs with predetermined weapon spawns (ie. skeletons with their bows) will not have that HandItem place overwritten with an item defined in the mob file. Mobs that do not have a predetermined weapon will properly obtain HandItems from Onslaught.

Corresponding Discord message: https://canary.discord.com/channels/620840315498004480/796434642038161428/826959324768894976

NuclearFarts pointed out that the behavior actually could be related to: onInitialSpawn which is overriding the gear.

Since the intention is to craft specific encounters- I may do some quick logic that if given an NBT tag with either HandItems or ArmorItems to avoid calling onInitialSpawn.