It's a problem with the cow potion specifically. As long as the cow potions regenerate themselves, the deck won't reset. The cow potions add a new potion whenever they combine (including higher level ones), so each potion is effectively worth 5 or 6. It's not realistically possible to avoid combining them long enough for the deck to reset, and of course once one pair combine it chains through a bunch of them.
I'm not really sure how to change it without losing the interesting part of needing to avoid combining them. Maybe put a limit of 3 in the deck at any time, or add a rule that maxing out their combination (to where the bottle breaks) removes all remaining ones from the cauldron and the deck.
EDIT: I didn't realize this until I was playing with slimes so I could actually play around the bovine elixir - even if you don't combine them it looks like they keep getting shuffled back in the deck. The bovine potions should definitely get removed from your deck once you play them and only get added back in when combined.