I see there is a topic posted about this for Nuendo 15, but I wanted to put it out there that the same issue is happening with Cubase 15.0.30.
I have a PLE to toggle a folder open/close, and if I try to toggle either a regular folder or a Group Folder that is nested inside another Group Folder, then the open/close toggle does not work.
I confirmed this by removing group function from any main folder. I assume this is a bug, any fixes in the mean time though?