Also, the name of the newly grouped tracks must be different than the name of the first track in the new group, otherwise it won’t work. So “Sweet Thing: Sweet Thing” must become something like “Sweet Thing Medley: Sweet Thing” when the TITLE tag is modified. It may work the first time, but once Roon is closed and then launched again, the individual tracks separate again.
EDIT: Apparently, this is more important than grouping alternate versions of the album – if the new group has a different name than the first track of the group, you can still group album versions while maintaining the grouping of the tracks within the album. I’ll edit/delete my bug report here: