Time Machine still backs up fine to HFS+ drives, and I'm having no problems with that. What it's not doing though is associating the new backup to the existing incremental backups. This is a core functionality of Time Machine.
So there's no problem attaching the drive and inheriting it and backing up to it. But the backup gets done as a separate volume, as if it's a new computer. In the tmutil man that's not what should be happening:
associatedisk mount_point snapshot_volume
Bind a volume store directory to the specified local disk, thereby reconfiguring the backup history. Requires root and Full Disk Access privileges.
In Mac OS X, HFS+ and APFS volumes have a persistent UUID that is assigned when the file system is created. Time Machine uses this identifier to make an association between a source volume and a volume store. Erasing the source volume creates a new file system on the disk, and the previous UUID is not retained. The new UUID causes the source volume -> volume store association to be broken. If one were just erasing the volume and starting over, it would likely be of no real consequence, and the new UUID would not be a concern; when erasing a volume in order to clone another volume to it, recreating the association may be desired.
A concrete example of when and how you would use associatedisk:
After having problems with a volume, you decide to erase it and manually restore its contents from a Time Machine backup or copy of another nature. (I.e., not via Time Machine System Restore or Migration Assistant.) On your next incremental backup, the data will be copied anew, as though none of it had been backed up before. Technically, it is true that the data has not been backed up, given the new UUID. However, this is probably not what you want Time Machine to do. You would then use associatedisk to reconfigure the backup so it appears that this volume has been backed up previously:
thermopylae:~ thoth$ sudo tmutil associatedisk [-a] "/Volumes/MyNewStuffDisk" "/Volumes/Chronoton/Backups.backupdb/thermopylae/Latest/MyStuff"
The result of the above command would associate the volume store MyStuff in the specified backup with the source volume MyNewStuffDisk. The volume store would also be renamed to match. The -a option tells associatedisk to find all volume stores in the same machine directory that match the identity of MyStuff, and then perform the association on all of them.
