Thank you! I do have that yoffset part in my code. When I toggle the achievements in the gallery, the multiple popups show stacking on each other just fine, but in the game, only one popup show up at a time .
Here is the code for granting two achievements at the same time, I think it works because the if A is unlocked and I have B, the C will get unlocked in the gallery, its just that C popup doesn't show in the game.
Currently, I'm using renpy 8.5.2
$ ending_A.grant() if ending_A.has() and ending_B.has(): $ ending_C.grant()