I had a similar problem - not sure if it's the same, but will share just in case. For me it was that I would move tabs to one of several Tab Groups I'd created in the past, but when I returned to those Tab Groups, the new tab I'd recently moved to the group (or even a new tab I opened directly in the group) had disappeared. The tabs that disappeared could not be found in history either. So I had 4 Tab Groups, all created months earlier, and all retained the tabs that had been put in them up until several months back. Then at some point more recently, any tabs newly moved to any of my 4 Tab Groups, would simply disappear shortly after having been moved. This issue is only happening to tabs added to or newly created in Tab Groups - and does not impact new tabs that are not in a group (Hope that description made sense.)
I searched like crazy for a solution but could not find one. The only things I found that felt like they might be in the right direction were referring to the issue as being somehow caused by the iCloud sync feature.
So I reviewed all of my Apple devices to see which had iCloud sync turned on for Safari, and that's when I realized that around the time my issue started was when I had set up a really old Mac mini (late 2014 model) as a cheap server used only for Time Machine on my main Mac. I still don't understand how this old Mac mini could be causing any issues, but I turned off Safari iCloud sync on that ancient Mac mini and my tabs stopped disappearing!!! Somehow, it seems, that old and super slow Mac mini being set to sync Safari with iCloud was impacting Safari on all my other Apple devices and causing tabs added to Tab Groups to disappear.
Can't explain why, but if you recently added an Apple device to your iCloud and it's syncing Safari, you might want to turn it off and see if the issue stops for you like it did for me. And sorry I have no suggestions for recovering lost tabs. I lost a ton of them too and feel that pain.