Permissions changing when multiple users access files via vnc/smb in shared (external) drives

macOS Monterey, v12.7.5, MacPro (late 2013). I use the MacPro with multiple users all assigned to a group with read/write permissions. This has worked for years without any problems with permissions, i.e. user 1 creates a file (in most cases on a connected external drive) and user 2 can modify/delete the file. Since the beginning of this year, this has changed fundamentally: Without a clear pattern, the permissions of individual users override the group permissions for the files/folders. All external drives are shared via the “sharing option" via smb (with Windows sharing enabled) with correctly set group permissions. External drives are formatted APFS or Mac OS Ext. (Journaled). Do you have any idea how to solve the problem?