Yes I have seen that before, however if you look even closer they are nipping at it softly. I can't speak for you, but when you saw this happen did you set the hook? Some do in your situation, I know I have in the past. You could swear they were going after it, and at the last second the bass bumps it and disappears. Keep your cool, and continue to work the frog and wait until you get a definite hit, then stick em. If they were constantly doing that, I would have switched to a fluke, and they probably would have taken those instead.