Hi Guys,
i also have this Problem since a while and unfortunately don't have any Solution for it though i've tried everything BUT... i maybe found a Workaround meanwhile Apple is fixing the Issue.
There is a simple free App called CloudyTabs which is very useful for this case.
It can be found here:
https://github.com/josh-/CloudyTabs
It stays in your MenuBar and shows you exactly what all of us are Missing: The open Tabs of all of our Devices. I had to grant full DiskAccess under Security in System Preferences of macOS Big Sur first to use it, otherwise the App won't open but after that i does what Apple can't since many Weeks. Unfortunately you can not close Tabs yet but it's better than nothing maybe. It thought it could be useful for you so i like to share this. Hopefully Apple fix this Issue soon.