Both you and I are guilty of crimping our thumbs, trying to
squeeze the ball to death sometimes. Over time, that results in
both abrasions at the top of the thumb (for me especially) as well
as the front tip (in your case).
Tip: When our thumb is in the ball, it should be relaxed and
straight. The only area of the thumb actually applying slight
presure in order to hold on to to the ball should be at the bottom
front (your nail would be considered "top back") section. Over time
you will actually develop a callous which will allow you to do this
without any discomfort.
If you feel you have to exert excessive force or crimp your thumb
just to hang on to the ball, it's either way too loose, or it would
be wise to get your ball fit examined. Absolutely no squeezing or
you will ruin your armswing!!