We had a problem recent where Veeam 9.5 which was installed on Windows 2012 R2 trying to restore from Exchange 2016 on Server 2016 with ReFS datastores.
It turns out, Server 2012 R2 cannot recognise ReFS 3.1 which is on Server 2016. From the screenshot below, this backup is missing two drives which are ReFS:
When logging in to the Exchange server, you will see the correct drives
To fix this you really only have one option: Install Veeam B&R on Server 2016, or at the bare minimum, the mount points on Server 2016.
Once you have a Server 2016 mount point, you will be able to see the ReFS drives.