I use Group Folders a lot and have no trouble syncing them with Windows 10 using the standard Nextcloud client. My guess is that your Group Folders also contain a lot of data, by default folders over 500mb are not synchronised until you enable them.
In the Windows client settings, you can change this default behaviour. Or you can simply select which folders you want to sync from your server.
The reason for this is that the Windows 10 Synchronises files, meaning they are stored in local folder on your PC and use your local disk space. This is different to the mobile clients which are browsing the online service, and only download a file if you choose to download/open it and so don’t require large amounts of storage.
I believe the group folders are considered external storage by the windows client.
There is only one option for this in the windows client settings “Ask before synching external storages”. If you uncheck this box it will synch all group folders automatically also new ones as they appear.
I finally tripped over the setting on our NextCloud instance which seems to have been responsible for the problems we were having.
There is a Settings area called “WorkFlow” which were were previously not using. That can be found at URL like xxxhttps://SOMESERVER/index.php/settings/admin/workflow
In that area the Admin can create a set of rules as to who can access what from where and when - seems a very powerful feature. There was a rule there saying “Deny access to any non-admin user who is using the Desktop Client”. Once I removed that rule it now seems to be fine.
Not clear to me what was reason for this rule being there.