The bug

Mobs never drop their armor or tools/weapons they are holding in their hands, unless they are killed by the player. They will only drop their tools/weapons if the player has the last hit on the mob.

This is probably working as intended. However, MC-99255 has been fixed, causing experience points and gear to be dropped even without player interaction in some cases.

To reproduce

1. Run

   /summon husk ~ ~ ~ {ArmorDropChances:[0.0f,0.0f,0.0f,1.0f],ArmorItems:[{},{},{},{id:"minecraft:iron_helmet",Count:1b}],HandDropChances:[1.0f,0.0f],HandItems:[{id:"minecraft:iron_sword",Count:1b},{}]}

2. Kill the husk with a sword
3. The husk always drops its helmet and sword. You can repeat this as often as you want
4. Run that command again
5. Kill the husk with /kill, or let it die on magma blocks, through fire, or through other means
6. The husk won't drop its gear

Video

Minecraft 1.14 2019-04-26 21-24-09_Trim.mp4