Is this a bug, or just an oversight?
Neither. The system is working as designed. Moderators are free to ignore (as valid or invalid) or act upon flags at their discretion, and may not always agree with you, each other, or previous or future versions of themselves as to how a flag should be dealt with.
Regardless, the validity of one flag at one point in time should not imply that previously-dismissed flags were valid or invalid. Situations change. Posts change. Perhaps not in this case, but often enough.
I still lost the 10 flag weight, though, even though an answer that I had originally flagged was responded to in the end.
So what? It's just flag weight - it doesn't actually matter, except to the moderators fishing the flags list. Keep flagging things that need to be flagged; you'll get it back.
Also, read this comment posted above:
"Not An Answer" is the most controversial of all flags. Moderator opinion has a high variance on this. If you are awfully concerned about your flag weight, then avoid flagging those. If you feel strongly about it, flag a post with "[x] other" instead and provide an undisputable reasoning as explanation. – mario
"Not an answer" is terse. It better be blatantly obvious that the answer isn't in order for the moderator to do anything about it - we don't need moderators running around deleting perfectly good answers just because someone didn't like them. Obviously, more users flagging will also bring more attention to a specific post - this is also by design.