If you add x amount of groups to a user the preview functionality stops working on pages.
The user will get the published version of the page in preview mode. If the page is not published the user will get a 404 page.
See screenshot of group setup then the functionality stops working:
If you remove the groups called g5 and g4 it starts working again. The number of groups when it stops working depends on the names/alias of the groups.
The functionality is tested on a blank solution of version 9.4.3 with no custom code.
We can have a live session if you like so I can show you more about the bug and exactly when it stops working.
This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/108860-save-and-preview-of-pages-not-working-when-having-x-amount-of-groups-on-a-user