Been silently watching the BC teams for a long time now.. here's what I know about w/o naming profiles to avoid another full lockdown of the related accounts:
-Testing have been happening again since the beginning of this year.
-The original team have gone dark around the time RDR was leaked, blocking their achievement and activity list. Now some changed to a new designated GT name or a random generated GT name.
-It's harder to pinpoint who's a BC tester and who isn't, since they mostly don't use a 100% designated name for the team anymore (was "[letter][number]test[number]").
-Furthermore (and as we all know), testing have been extended to Xbox one games to ensure they're working on Xbox Series consoles.
-Unlisted 360 games are mixed between Xbox One releases because of this. But you can notice that there was a long pause between the older BC tested games -> games being tested before the Series launch/newer games released in-between -> newer BC titles.
-A lot of japan-centric games (shmups, musou, etc), with achievement activity as recent as 19 days ago. I believe one of the reasons is to extend games available on game pass to attract users from that region.
-There are some EA games in the mix, maybe to extend the EA Play catalog. Some may be part of the older BC testing program tho, since they're probably a licensing nightmare.
-There are some interesting titles in there that I see repeating on different testing profiles. Especially considering the Bethesda buyout and so on.
-OG X titles are harder to discover, since they aren't listed by XBL in user profiles. The only way to discover them is through store listing leaks, like the one reported by Shpeshal Nick and TA.
So yes, there is something going on behind the scenes. Hope we get to see what the BC team is cooking soon!