Steps to Reproduce: 

Go into creative gamemode

 Place a repeating unconditional always active command block with the command /execute if entity @e[name=<GamerTag>,nbt={Inventory:[

]}] run clear <GamerTag>

Place a chain command block that is conditional and always active that the repeating command block leads towards with the command /give <GamerTag> stone

 Get one piece of stone and place it in your inventory

Open your inventory and go to the survival inventory tab.  

While hovering over stone block, press the swap item with offhand button twice

Type /data get entity <GamerTag> Inventory   Into your chat

Observed Results: 

Your inventory shows you have a piece of stone in your offhand but the /data command says you have a piece of stone in Slot:0 or your first slot

Expected Results: 

The piece of stone should be shown in your first slot and not your offhand

Notes: 

You can do this bug with any item just change which item the command block gives you.  Once you are in this state you can select any slot except for your first one in your Hotbar and when you try to place your offhand block down it appears then instantly disappears.