I think thats right. There was an article in PBO about new sails on an SS26 a few years ago which reached the same conclusion.
Inadequate mast rake may contribute too.
Why it should happen now is another question, probably because of the "power" of the new sail.
I find on Cascada I get lee helm more often with the 100% blade than with the Genny, but put it down to the centre of effort of the sail plan being further forward of the centre of lateral resistance of the hull, with the blade, as opposed to the genny.
In any event in light winds after tacking (and before speed is picked up and the keel starts to generate lift), I often experience heavy lee helm. A dirty bottom does not help either!
As flow over the keel picks up and lift is generated the keel starts to "work" again, and the lee helm usually disappears
275/30 "Cascada"