there is a bug in WP Smush PRO.
When you set "Automatically smush my images on upload" to "off", list of thumbnail sizes disappears. But this list is for manual smushing images too so it shouldn't disappear.
Better it should not be in section "Automatically smush my images on upload" but in new separate section.
And there is another problem. When you have some thumbnail sizes unchecked and some images were smushed and then you check all thumbnail sizes and save settings - "recheck images" finds nothing and those thumbnail sizes are unsmushed and there is no way to smush it.