the advanced permission rule dialog for groupfolders in the sharing sidebar is often disappearing. You have to reload the page (often several times) for it to reappear. I was able to reproduce this on two different Nextcloud instances.
I have this problem also (v18.0.4) with normal account.
Sometimes reload page or open/close sidebar helps.
I run it on a raspberry. So maybe its a timeout problem?
We have the same trouble (administrator and user) with version 18.0.4. For the moment I haven’t identify any pattern to make the advanced rules appear systematically.
Anyone willing to debug, since it’s randomly happening, I found the dialog visibility is conditioned by a param returned by the PROPFIND call. When this is happening it could be an idea to check if the param is correctly returned. On the issue a dev is also asking if there are browser console errors when the dialog is not visible.