Just to say how I resolved my case, most probably temporarily. An iPhone with latest iOS and an old MacBook Pro with Catalina. Never had issues, apart from the last 1.5 year, which seems to be the same case for most people.
Was trying signing out of iCloud, toggle off and on the Safari tab but nothing. During the last month, I also lost syncing of bookmarks, reading list etc. and not only iCloud tabs.
Don't know if it was a coincidence, but located an option I never saw before "Use Start Page on All Devices". It appears when you create a new tab at the bottom right settings -> see attached in MacBook, and under Edit in new tab of iPhone.
After I disabled it completely from both devices, I tried one last time to toggle off the Safari from the iCloud settings in the iPhone, waited a minute and enabled it again. Suddenly and magically everything re-synced and a couple of ghost tabs went away as well.