Safari 15.0 tab groups lost after each restart
Safari 15.0 seems to have trouble syncing tab groups with iCloud or retaining its current status. I took some time to set up several tab groups, but each time Safari or my Mac are restarted it reverts back to the full set of open tabs that I had before updating to 15.0 and the tab groups sidebar shows no tab groups created yet. When left loose, the open tabs show as iCloud options on my iPhone (iOS 15), but once groups there are none shown on my phone. Tab groups also aren't syncing across devices, which I think is how it's supposed to work.
Any suggestions for a library file that might need deleted to free this up to work correctly, without risking losing my current set of open tabs or my bookmarks? Or other solution? Or maybe it's just a straight up bug that will have to be fixed in a future Safari update.
FWIW, I generally like the changes in 15.0 so far (although restoring the option to pin tabs within a group would be highly desirable).
Mac mini, macOS 11.6