I finally figured out why my endpapers kept bubbling
Craft paper, PVA, all the right weights. Still got waves every single time. Three weeks of ruining 15 books before it clicked. The issue was the paste I mixed, too thin and it soaked straight through. Switched to a wheat paste and PVA blend at 50/50. Pressed overnight with wax paper between every section. Clean flat result, first try after that. Anyone else fight with endpaper bubbles or did I just miss the obvious step?