Thanks all for the replies:
1) it does not happen all the time.
2) not to the same key.
3) Some suggested that maybe it is when it the busiest... that might be true. I noticed also that after playing with styles.
4) I tried so many things and the only thing that works for sure is to play many layered keys together to make polyphony run away asap.
5) the problem is that I cannot reproduce it at all. It just happens without expectance.
shall we ochrestrate a move to bug GEM to fix it?
And by the way, I am bothered by the fact that it did not happen to me ... not even once... in the last six months!!!