I have the same problem. It seems that morphs of face can escalate expression morphs.That is not bad at all! I always have problems with the Daz models looking nothing like the expression thumbnails, they are always grotesquely misshaped. I wonder if I'm doing something wrong and have to apply something first.
Lately I'm using 50% of expression, gives a lot better effect. Of course, if expression is "scream" it will not be that "powerful", but lot more natural. And I'm often playing with Lips Part morph to tune the expression.