I have used this add-on for years and love it! Recently (I think since Firefox update 132.0.1) all the tabs keep deleting whenever the last tab of a group is closed. I hope this gets fixed, but for now there is a temporary workaround:
In about:config you can set `browser.tabs.closeWindowWithLastTab` to `false`. (There is a better explanation on the github-issues page, under issue #1229 but I can't paste URLs here.)