".\<album-artist:1>\<album-artist>\<filename>" or so.
With ok the file is moved properly
returning to the dialog the just entered mask appears back in a changed manner:
".\-1R\<album-artist>\<filename>"
which obviously won't work as originally desired.
This problem doesn't appear when using "<artist:1>\<artist>\..." instead.
BTW: the length modifying option is really great for doing things like this.
(MM version 2.2.2.780, but with german UI, thus the text reads "<album-interpret")