Not a real workaround, but the behavior only happens on release — if you hold Option while dragging the group, Finder keeps the relative offsets and drops them where the mouse ends. Same as the old copy-with-preserved-layout trick. Doesn't help if you're moving them across displays or Spaces, but at least the mount volumes stop collapsing into a stack for me.
Filed feedback FB about it during the beta and pointed at the diff vs 26 — got the usual "thanks, tracking" reply. Wouldn't be surprised if it's actually an intentional change tied to the new grouped-icon draggable in 27 rather than a bug per se.