you're right, and it was a real bug on our side. "include tab groups" was off by default, and with it off a save skipped grouped tabs completely - they weren't saved and weren't closed, so saving a window looked like it ignored half of it.
fixed, ships in the next version. grouped tabs get saved with their name and colour and restored back into a group.
if you want it working today: settings, tab groups section, turn on include tab groups. that's the switch.
