Big Sur 11.1, 6.0.1b48, TB4
When an APFS volume is created, then it shows up as mounted in the finder, but the volumes list thinks it is unmounted.
If you then go to create another APFS volume, then after the drives are selected, Softraid says that it cannot do that because the disk in use for a different application.
It turns out that you need to go into the finder, eject the APFS volume, and then you can create the new APFS volume; it will then mount both the old APFS volume and the new one.
That is correct. The current beta does not support APFS volumes in the user interface yet. So you need to manually unmount all volumes first.
I have noticed that in some cases I was unable to do some volume operations (especially deletion of volumes) unless I had unmounted all of the other volumes on the same physical drive.
Unfortunately I cannot test that rigorously at the moment, as I am currently unable to delete any volumes (I opened a case about that and sent the support file already.)

