According to the documentation after backup is moved from old to the new (refs 3) repository in order to use fast clone compact operation should be done.
In order to our ReFS integration to take place, new active full backup is needed. This requirement can be met by running either active or compact full backup.\
I can for sure deselect synthetic full and select active full or compact, but if I have 100 tasks, it became not practical.
This can be accomplished via PowerShell. If you need assistance with it, feel free to create a separate topic in the corresponding subforum.