Just making a topic for this because I screwed up testing it without taking photos. I had 4 stacks of seeds, 20 5 5 5. All were above Q10. I took the 20, right clicked it onto the five. Did this for the other stacks (each time stacking the larger seeds onto the small seeds). When I was done, I had double quality seeds. I split the stack, and the new stack didn't have double quality. When I stacked the double quality onto the new stack, the whole thing was back to single quality.
Weirdly though, my Q10+ grapeseeds are now 7/5/5 in quality...
At least I remembered the steps I took to cause it