Comment # 1 on bug 1216110 from Neil Rickert

Yes, I have run into this.

I have Leap 15.5 as my main desktop, with Tumbleweed installed in a different
partition (actually a different LVM volume).  And I cannot access the
Tumbleweed root file system from Leap.  And Tumbleweed does not show up in the
normal grub boot menu for 15.5.