1. Put one of any pylon in your inventory.
2. Don't have any items positioned after it in your inventory.
3. Your offhand slot does this:
   
   If you pick up the item with your mouse while under a potion effect, the potion UI does this:
   

Version / modlist is the same as in the current development instance, so this does include Thaumcraft.

Checked and this happens with no Thaumcraft too.

This issue is stale because it has been open for a while with no activity. Remove stale label or comment or this will be closed soon.

Still a thing in 1.14