The good news is there is a solution to this which allows you to get around the problem. You’d have to generate persistent shares for each user.
This allows you to maintain ownership of their files even after they’ve been removed from your group. Nextcloud also allows you to enable permanent file shares by creating a special shared folder which you can then share with select groups and/or users.
To ensure the files stay intact, create user permissions so that no can take files with them when they leave the group. It’s worth noting that whoever shared the file remains its owner regardless of who edits or shares it afterward.