WinRE ending up with a different version of fstx.dll in it seems like a pretty standard Microsoft (or any other big company) thing to have happen? Again, it all comes down to whether you think the drift was a malicious internal fork or a simple mistake. I will say that the functionality being different makes it an inferior backdoor in many ways; especially in Windows land vulnerability researchers are obsessed with binary diffing, and any delta internally would be more likely to be discovered as a backdoor in review too (ie - “hey maybe we should update fstx in winrt finally, let’s review the drift to make sure there’s not going to be a regression, wait a second why did xyz employee add this suspicious looking code”).

A fun next step would be to look at different fstx versions to see if it’s just something that was patched or refactored out at some point. At that point it could be a patch-door (ie an organic bug where the patch was held back by interference), but again, that would be a crappy setup due to the propensity for Windows vulnerability engineers to use binary diffing - if you had the exploit and the power to hold back the patch, it would be way better to hold it back everywhere.

I'm not necessarily suggesting they intentionally made the dll different for RE. The possibility that RE was maliciously backdoored is certainly possible, but there are three plausible other possibilities I can see:

1. A bug was introduced that affects both, and the bug never make it back into the 11 branch

2. There's conditional logic in RE that triggers the issue

3. 11 introduced new behavior that never make it to RE, causing the bug

The fact that 10 is seemingly unaffected is telling. #2 seems very unlikely, because it suggests new conditional logic was added and not tested. #3 seems unlikely because I can't understand why the binaries would be different anyway. #1 seems unusual because it suggests there's no canonical source of truth for the code, which feels very unlikely for bitlocker of all things (where you want everything speaking the same language).

If there's any benign explanation, I suspect it's likely due to incompetence. This feels like such a strange problem to have. I suspect the follow-ups you suggest are going to happen very soon and we'll know more.