Haven't finish the game yet, but i noticed there is a 'bug' where you can't see 'items' throught this glass walls, if you look closely, you can also see that the walls become invisible too. If that is an intended mechanic, i'm not really fond of it.
Viewing post in Celeste 64: the bee-sides comments
it’s an unfortunate behavior of non breakable glass. It’s probably easy to fix when you dive in the code but haven’t done that because I wanted the mod to be non destructive to the main game.
I’ll look into fixing it in the Fuji version in a few weeks, but can’t make any promises.
Fortunately it’s only happening in two cassette levels, but I do agree it looks weird.