This is a replicable bug. I ran the command "/execute if entity @s[nbt={Inventory:[

]}] run say hi". This checks if the slot 103 is filled with an item, and if it does, it says "hi". I ran this command with my helmet on, then with my helmet off, then with my helmet on again. On the third time, I got kicked for "Chat Message Validation Failure". This happened on a vanilla server without any anti chat report mods that I know of (Self hosted). This works with an indefinite amount of helmet on commands, and indefinite amount of helmet off commands, always on the third run, I get kicked.