More likely Bitdefender being overzealous than a bug on XF's side. I have Bitdefender on my work laptop and I am pretty sure it gave me an option to unblock the site a while back. Only time I have seen it complain and I am on four Xenforo sites including this one.
It's not a bug; I don't have Bitdefender to be able to test but I'll submit to the behaviours being reported here which imply it's far too intermittent or conditional on other factors to be a bug.
It would also seem the same behaviour isn't exhibited here or other sites which implies it's site specific.
People running into this may need to add an exception for your site in Bitdefender, but ideally you need to try and locate what specifically about your site has been flagged up. Is it some specific content? The ad network? An IP address from your host that has been flagged as spammy? Some other false positive? I don't know what the process is but you may have to try and contact Bitdefender themselves.