This is a really good game, although the most glaring issue is being unable to save your progress between sessions. This would be fine if it were a shorter or more arcade-styled game, but for a game with this much progress to be saved, not being able to do so is a big issue. Especially with the bug that has something to do with a slime attacking you at the same time you're trying to shoot. If that bug happens, you turn invisible, can't move, and have to quit the game to keep playing. Additionally, the fact that your attacks just have a chance to not fire is a bit frustrating, especially since that fact isn't communicated to the player. If you were to make a save feature, even an annoying manual one that uses text strings or something, this game would be 5 times better. I hope you continue developing this and/or make more games, since your visual style is very good IMO.
so the bugs you mentioned are all due to the refresh rate of the game, you have to set it to 60hz I believe? been a little while but lowering it to that fixes all issues you have had listed. The game is roughly 15 minutes when you know what you're doing with the refresh rate set correctly, hopefully that helps!
the game doesn't have a save feature since its so short (roughly 10-15 minutes), go into your computer's display settings -> advanced display settings -> choose refresh rate to 60hz. This will stop the slimes for bugging you out, casting spells no longer will fail and you can move much much faster (if you don't change the refresh rate getting the item from the bunny race is impossible as even with all the mushroom speed upgrades you won't be able to win).
I changed my refresh rate, and it worked well on making movement a lot more fluid, and it fixed the spells not firing bug. But, I still get hit by the vanishing out of existence when hit by a slime while charging a spell bug. Maybe you could try adding an "unstuck" feature in the menu that just kills you and sends you back to your last used checkpoint.