Replaced mainhand when mainhand is dropped using repeating command blocks or functions

What I expected to happen was...:
item is dropped and main hand is replaced

What actually happened was...:
on client, item is dropped but main hand is empty
on server, item is dropped and main hand is replaced

Steps to Reproduce:
1. set up command block chain
2. command1: execute if entity @e[type=item,nbt=\{Item:{id:"minecraft:wooden_pickaxe"}}] run item entity @p weapon.mainhand replace minecraft:wooden_pickaxe

3. command2: kill @e[type=item,nbt=\{Item:{id:"minecraft:wooden_pickaxe"}}]
4. get a wood pickaxe, drop and observe

command2 simply exists to ensure item replace only occurs once. this can also be done through other means.