Keep an eye on those. I’ve had Tab Groups get out of sync between my devices several times, too.I’ve decided to move to tab groups entirely (meaning I don’t use the on-device tabs anymore).
I have a personal Focus mode that’s enabled when I’m not working. I configured the focus to use a tab group also called Personal on every device (you could configure separate tab group for each device if you don’t want the same tabs open on every one).
Then I moved all my "normal" tabs from every device to my Personal tab group (making the local group empty). It took a while, but now my iCloud tabs are empty on all devices (not that I would care since you can just turn off the iCloud Tabs section on the start page) and no more phantom tabs.
Haven’t looked back since.