If (Q, B) is really the key resolved to by your key selector for the end of the range, then it shouldn’t be part of the conflict range (conflict ranges are exclusive of the end key). If your selector happens to resolve to a key after (Q, B), though, then maybe it’s possible an update to (Q, B) could conflict.
I would actually expect, though, that the conflict range for your query to end at (Q, A).\xFF regardless of what the end selector resolves to. This is because the answer to the query doesn’t depend on anything after (or including) that key. If your conflict range is larger than that, that may indicate a bug.
If you are having conflicts of that nature, then a workaround could be to use snapshot reads and then manually set the conflict range to end at an appropriate key. It might also be a good test to see if the conflict ranges are actually larger than necessary or if there is a different conflict involved here.