Opus only tracks the last active tab. If you run go tabselect=prev repeatedly, it'll toggle between two tabs. Each time the command runs, the tab which was active becomes the previously active tab, and the previously active tab becomes the new active tab.
There's go tabselect=+1 and -1 for that, but I agree the two are easily confused if you don't look in the manual. I think earlier/later could be as well though (maybe less, but still), since most words relative position are also used for relative time.