Grab all 3 of the colored ones from the colored portions. They'll appear at the top right of the screen (like in the screenshots) when you've collected them.
Viewing post in Gedou Night 2 jam comments
Ah, apologies. At least I can say there's no gameplay after that; just credits.
Could you hand me your save file? Can open it in a text editor and put on pastebin or something.
EDIT: Wait, I found the bug: Dying doesn't remove the squids from appearing on the UI, but DOES remove their collected flag. So that will mislead people into thinking they've collected them all.
If anyone else has problems opening the door, then I suggest saving and restarting the game. Then you should see which squids you need to collect again.