Hi Cornfab, thank you for playing our game :)
Only 'mixable' (or items with recipes) can be dragged into the Mixing Table, which to my knowledge are Orange Potion, Blue Potion, Gold Potion, Harmonious Potion, Mace and Falchion.
I have not encountered this bug with drag and drop into the Mixing Table before, but I'll keep an eye out. Changing level should not affect that part of the UI.