Strange.
Is it happening on a specific adventurer? Are you sure they aren’t waiting for you to give them something?
Have you tried picking up and putting the cake back down an extra time? Maybe kick it out of its weird state.
I can't get past the second adventurer, Violet, but I've also had this happen with the adventurer before, Glamdalf, when I'm not quick enough to make it. I can interact with everything else just fine, but no matter how many times I re-interact with the cake and no matter what I try to give the customer, the potion machine simply ceases to be interactable.