I just upgraded to Big Sur and did a fresh install. I installed Core Tunnel and restored my premium purchase, made sure Sync was set to iCloud but none of my tunnels restored.
Luckily, I have a bootable backup of my Catalina partition. I booted it and sure enough, it was saying everything was synced. So I made some duplicates of my tunnels from my Catalina install, thinking that might force the sync. I rebooted into Big Sur, but nothing appeared in the tunnel list.
So I exported my tunnels manually from my Catalina install and imported them into my Big Sur install, but I really don't trust that they are actually syncing. I also just now tried switching from iCloud sync to Folder sync. I created a new folder and checked that it said "Last synced: now"... Unfortunately, the folder is empty, so I don't really believe it.
Does sync work?