Actually, yes, apparently it is normal. It was something added to the 1.02 patch of LOMAC, as apparently the real radar has this limitation as well.
Taken from the 1.02 readme.
It makes sense that you would really only notice it during BVR, because since 1.02, the EOS automatically attempts to maintain lock on the enemy aircraft if the radar loses lock for some reason. Obviously, EOS will be unable to maintain a lock at any great distance, but in WVR, it stands a good chance.