I have the same problem, CG shift doesn't seem to make a difference. I glued a piece of leather to the inside of the clip which holds the trim idler pulley. This places friction on the pulley stopping the creep. At least we know that your trim system is functioning properly without any jamming or rust in the stabilizer torque tube.
On the trip home from buying my Super Cub it did the same thing. Never has done it again but I started changing things like tires, prop, gear etc. and it has never done it again.