Yes the databases (source and destination) are the same version. I have checked 3 times. Both are "Microsoft SQL Server 2019 (RTM-CU8-GDR) (KB4583459) - 15.0.4083.2 (X64)"
The database itself restores ok without error. Only when I try the read-only file group restore does it give the error. I have redone the back up and attempted the restore again just in case I had something wrong with the backup.
I have done this numerous times with ss 2016 and even when upgrading from ss2016 to ss2019. But now I am testing the restores it just looks broken to me. Almost like this is a bug