For sure, get that thermometer right at the ribbons and see if there is a temp difference. I can only speculate but it seems that water temp and sun exposure is part of the equation but I had ribbons in the mesh of one of my taller pyramid/dome style minnow nets. It was as if the perch wanted to swim into or over something that the eggs could get snagged on and help pull them out? I wonder if that is why the assumption is that branches are what they are preferring since some ribbons are found draped on branches. I wonder if somehow the oak leaves are stiff enough that they can snag the ribbon on the leaf and wiggle around and have the ribbon come out better. Imagine having to get that huge ribbon out of a tiny hole and you might wish you had a suction pump or something to snag it on so you could swim the opposite way and get the egg laying over with in a hurry too..

Bill C might know if the GSH eat the eggs before they hatch (never heard that) or if the fry are consumed by the GSH.

It would be fairly easy to put a mesh around a few egg ribbons to see if protecting them helps.