On Castle Town, if u get an apple and move it to a different slot than the default (idk if a lot of people do it but I like to put them in the last slot) and get another apple, it’ll stack with the first one. It works for apples on Nectar Lake and cupcakes on Backrooms too.
On Spaceship, tho, if u get a potion, move it to a diff slot, and get another one, the second one doesn’t stack and just goes to the 4th (?) slot instead. I’m not sure if it’s a bug or not. My suggestion is that the spaceship potions should stack too even if u move the first one to a diff slot than the default. I’m assuming it’s not already a thing on Spaceship because Spaceship is an older map and they probably just hadn’t decided to add that feature then. This is my first suggestion; hope I did it right haha