Mobs summoned by the player with the summon command with equipment continue to render the item if it's removed via command in the same tick.

Steps to reproduce:

• place a command block with the command:

  /item replace entity @e[type=minecraft:zombie] weapon.mainhand with minecraft:air
  

  and set it to Repeat and Always Active
   

• place a second command block with the command:

  /summon minecraft:zombie ~ ~1 ~ {HandItems: [{id: "minecraft:diamond", Count: 1b}], NoAI: 1b}

• Activate the second command block with redstone

-> The diamond in the zombie's hand disappears as soon as the zombie spawns

• Run the same command from the second command block as player

-> The diamond is still rendered despite being removed from the zombie's hand (Check with /data get)

• Relog

-> The diamond is no longer rendered