JC, supposedly (I don't have W8.1):
"Microsoft claims that malware is “almost designed to talk to world,” so that’s a main weakness that could allow anti-virus software to detect and block it.
As a result, Windows Defender now comes with high-performance behavior monitoring, so the application automatically and anonymously sends an activity log to the cloud for analysis. Signatures may be issued only a few minutes later to make sure that you’re 100 percent protected.
Windows Defender can now identify malicious patterns based on behavior, which means that it could block malware aimed at files, registry, processes, threads, and network. Windows Defender is the built-in anti-virus software offered to Windows 8 and 8.1 users and it cannot be uninstalled. The app is automatically disabled once you install a third-party security app and turned back on once you get rid of it." - http://news.softpedia.com/news/Microsoft-Adds-Windows-Defender-Security-Features-in-Windows-8-1-365175.shtml
On the other hand, I found this which contradicts the quote above:
"Microsoft’s first priority, however, is to maintain the relationship that a user has struck with the third-party anti-malware provider, said Holly Stewart, the senior program manager from the Microsoft Malware Protection Center. “We have to work collaboratvely across the industry,” she said.
”As a customer goes into an unprotected state, we want those antivirus vendors to be installed as the first upgrade source,” Stewart said. If the license has expired, the first thing Microsoft asks them to do is to go upgrade, she said."
To me, that looks like MS is trying to be noncompetitive, which isn't exactly reassuring (at least to me).
Not sure this really answers anything for you...
Did you discuss your problems with BitDefender or look in their Forums? Sounds like a configuration issue...