I'm experiencing the same thing here...
I think the title of this issue can be renamed, as the "requirements" are as shown by @Jimpaly very low, and no tab groups or similar is needed. A suggestion for name is "Tab order randomized upon restart/window reopen"?
On my end, I have about 40 tabs open with pages I visit semi-periodically, hence why I'm so bad at closing them. All of them in the same tab group, I've not yet figured out a good way of organizing them to my liking. 😀
Over the time I've noticed small changes in the order, but not fully realized what is happening until today.
Prior to a reboot I was visiting the Swedish IRS web page (tax declaration season over here), with two tabs open lined up neatly next to each other, and I was also reading a bunch of threads on SmallNetBuilder Forums.
After the MacOS reboot and Orion starting back up, the tab order came out like this:
The two tabs originally placed next to each other was now "split" by the forum tabs, giving a clear proof that the order of the tabs are mangled.