I have faced the same issue. In my case the scenario was:
- iPad with iOS 14.X & 15.X
- iPhone with iOS 14.X & 15.X
- Macbook with MacOS 11.X
Safari was completely updated in the three devices.
I'm dragging this issue since iOS 14 and I've finally solved in iOS 15. I don't think that the issue is related to iOS or MacOS itself. I think that the issue is related to iCloud synchronisation.
In my scenario, both the iPad and the iPhone were showing "ghost" tabs from the Macbook that are already close. I've tried a bunch of different solutions, some of them found in this question.
After some tries, the final solution was:
1.- Completely close Safari in the three devices (even remove the app from the multitask)
2.- Disable Safari synchronisation in the Macbook
For this step, you have to go to "System Preferences > Apple ID" and untick Safari.
3.- Remove all files and folders related to Safari under your user's "Library". These files and folders are:
- ~/Library/Caches/com.apple.parsecd/CustomFeedback/SafariAutoPlay (folder)
- ~/Library/Caches/com.apple.parsecd/CustomFeedback/SafariAutoFill (folder)
- ~/Library/Caches/com.apple.Safari (folder)
- ~/Library/SafariSafeBrowsing (folder)
- ~/Library/Containers/com.apple.share.System.add-to-safari-reading-list (folder)
- ~/Library/Containers/com.apple.Safari.SafariQuickLookPreview (folder)
- ~/Library/Containers/com.apple.Safari.DiagnosticExtension (folder)
- ~/Library/Containers/com.apple.Safari.CacheDeleteExtension (folder)
- ~/Library/Containers/com.apple.Safari (folder)
- ~/Library/Safari (folder)
- ~/Library/Mobile Documents/iCloud~com~apple~mobilesafari (folder) [Note: After removing this, it will be automatically recreated]
- ~/Library/Mobile Documents/com~apple~SafariShared~History (folder) [Note: After removing this, it will be automatically recreated]
- ~/Library/SafariSandboxBroker (folder)
- ~/Library/Application Scripts/com.apple.share.System.add-to-safari-reading-list (folder)
- ~/Library/Application Scripts/com.apple.Safari.SafariQuickLookPreview (folder)
- ~/Library/Application Scripts/com.apple.Safari.DiagnosticExtension (folder)
- ~/Library/Application Scripts/com.apple.Safari.CacheDeleteExtension (folder)
- ~/Library/Application Scripts/com.apple.Safari (folder)
- ~/Library/Preferences/com.apple.Safari.PasswordBreachAgent.plist (file)
- ~/Library/Preferences/com.apple.Safari.SandboxBroker.plist (file)
- ~/Library/Preferences/com.apple.Safari.SafeBrowsing.plist (file)
- ~/Library/Preferences/com.apple.SafariBookmarksSyncAgent.plist (file)
- ~/Library/Preferences/com.apple.Safari.plist (file)
- The symbol "~/" means your user folder. Don't search those files and folders in the system's Library.
- At the end of every file or folder, between parenthesis, is the type of element that you're looking for. File or folder.
- Also at the end of some lines, between square brackets, there are some notes about the file or folder.
4.- Open Safari in the Macbook
In this step, some of the previously deleted files and folders are going to be created again, but with the history, bookmarks, reading list, open tabs, etc. with no data. Those files and folders will contain a brand new Safari configuration.
5.- Enable Safari synchronisation in the Macbook
For this step, you have to go to "System Preferences > Apple ID" and tick Safari.
6.- Wait until all your bookmarks, history, reading list is synchronised
7.- In this step I've configured Safari again. Things like "Search engine", "How Safari opens", "Enable extensions", etc.
8.- Open Safari in the iPad and in the iPhone
At this point the "ghosts" tabs disappear and never appear again. At least in the last 5 days that I've waited between doing the procedure and writing this answer.
Hope someone finds this useful.