Yeah, doesn't sound like a great thing to me either (unless maybe you're owning your opponents). There is only one reality in combat, and that's immediate reality. If you aren't fully tuned into that reality, how can you hope to deal with/react to it effectively?
Do you also get things like tunnel vision, the sensation that things are moving in slow motion and other effects of high amounts of adrenaline?
Experiencing fear during your training is good for actually preparing you for real world combat. But, completely blanking out isn't good either.
Perhaps you just aren't mentally/emotionally ready for full on hard sparring or fights. Different people will be at different levels as far as what drills/training types they are ready to deal with mentally/emotionally.
Maybe work on some drills (other than sparring/fighting) where you are also under high amounts of stress and see if the same thing happens. If you are, then maybe try lessening the level of stress until this doesn't happen and then gradually increase as you become more and more comfortable.
In other words, work on your mental/emotional conditioning and perhaps fear management.