Thank you for letting me know about this issue and sorry that you're unable to progress. The slime should indeed turn into a smaller slime upon killing it.
I'd recommend restarting the game as this bug is (to my understanding) quite rare so I've been unable to replicate it myself to find the source of the problem. Fortunately the game is quite short at this stage.
I'm considering reworking the enemy system entirely to prevent this from happening.
Happy holidays!