adding more context (we are playing together) we managed to get into the mirror room well before meeting with Rosa and completed the puzzle, but nothing happened at the time. After talking with her and coming back, we have triple checked through multiple walkthroughs we did complete the puzzle, but talking to the mirror will not open so we can continue. scrolling down it seems two others had the same issue without a solution :(
edit: we decided to just completely restart the game and upon the restart the room was locked, unlike the first playthrough. it seems like some bug that makes it unlocked before you are supposed to actually be there, causing the softlock.