Are you talking about the old RS4 sail or the newer RSS MK IV ?
I suppose the latter.
I haven't heard yet about mast breaking problems with the MK IV.
I confirm that I've heard a lot of these problems with the older RS4 sails. They seemed to put an awful lot of pressure on the mast. Also non NP masts broke in these sails. Especially the larger masts in the bigger sails.
What sail sizes are you talking about ?
IS87 / IS101 / IS122 / US147