I'm not sure what to say. I've long felt that a virtue of cork grips is that it offers a secure grip under all weather conditions, including heavy rain. Being a life-long western WA resident, I've fished in the rain a lot. I haven't experienced the grip-slip that you mention. Admittedly, 1/2" in a day is getting up there in the realm of serious rain and is probably close to the threshold where our rivers blow out, and I call it a day and wait for the rivers to drop back into fishable shape.
Sg
Sg