When using /replaceitem on the offhand or any of the armor slots while having a container, like an ender chest, open, the item won't be displayed client-side.

How to reproduce
1. Place a repeating command block pointing into a chain one

2. Put the following commands in the repeating command block and the chain command blocks respectively:

execute as @a unless entity @s[nbt={EnderItems:[{id:"minecraft:stone"}]}] run replaceitem entity @s weapon.offhand minecraft:dirt

execute as @a unless entity @s[nbt={EnderItems:[{id:"minecraft:stone"}]}] run replaceitem entity @s enderchest.0 minecraft:stone

3. Take the stone block off the ender chest and notice that the dirt block isn't in your offhand.