Oh. This is just so. darn. painful.
I have around 300 tabs open. I accidently open one in side view when I’m just moving the tab order and it decides its “side view time”. I’m panicking and merging all windows by long-tapping the 2-square icon in the damned sideview. It’s all back, pfew, I relax. And then I come back to safari a few minutes later (my guess - it loads off from memory when I use something else). And then there is only one window with freaking TWO tabs from my old window. There are definitely no other windows found by bringing the dock up and tapping the safari icon. And there’s no offer to restore the last closed safari window on that screen. There are none of the lost tabs in the “recently closed” list via long-tapping the plus sign in the current window either, just the regularly closed tabs.
Just back up your tabs, guys. Don’t repeat my mistakes. Especially, back up every time you do a side or split view, via the bookmark all open tabs option.
If I’ll figure out the exact pattern that reproduces the issue every time, I’ll surely bring it up to Apple (what is the most efficient way?). But it saddens me the issue has been there for years.
I also wonder, if I had any root access, would I find files like session.backup, like with some desktop browsers? Couldn’t google that info at all.