That worked. No more FOUC, and the image lazy-load is active again!
That did it! Un-checking “Lazy-load images?” under “Image optimization” removed the FOUC while pages load.
Disabling JS optimization didn’t seem to change anything when I tried. Also tried separately disabling CSS optimization to see if that did anything and nothing changed. May seem like a silly question – should I clear my AO cache when I make these changes to properly test?