When a mob spawns with an item that is then immediately removed, the item will still be shown visually.

Steps to reproduce

1. Freeze the tick rate

   /tick freeze

2. Give yourself a zombie spawn egg with armor (this is just for testing, it also works with natural spawns)

   /give @p zombie_spawn_egg[entity_data={id:"minecraft:zombie", HandItems:[{id:"minecraft:golden_sword",count:1},{}], ArmorItems:[{},{},{},{id:"minecraft:golden_helmet",count:1}]}] 1

3. Spawn the zombie
4. Remove the items

   /data merge entity @e[type=minecraft:zombie, sort=nearest, limit=1] {ArmorItems:[{},{},{},{}],HandItems:[{},{}]}

5. Unfreeze the tick rate

   /tick unfreeze

6. The zombie will burn in the sunlight and according to /data is not holding any items

   /data get entity @e[type=minecraft:zombie, sort=nearest, limit=1] ArmorItems
   /data get entity @e[type=minecraft:zombie, sort=nearest, limit=1] HandItems