Safari tab group and bookmarks disappeared after ios 18 update

If you have backups of your home folder, restore the following system folder:

~/Library/Containers/com.apple.Safari/Data/Library/Safari

to the version where you had your tab groups.

You might need to do it from the command line....

Most probably, the problem is due to iCloud sync, so before doing this, it is a good idea to quit Safari and switch off iCloud sync for Safari in System Settings > your account > iCloud > Save to iCloud

Hope this will help!