Does it have to do with the "Smart Scan" feature I heard some years ago, which caches "known clean" items?
If an infected file is not caught by BitDefender AV (because it's not in the definitions yet and heuristics didn't catch it), and hence is added to the cache of clean files, and then a definition update happens which allows for detection, will BitDefender AV "skip" the file, allowing it to remain under the radar?