I don’t know if there’s a way to do this in the regular UI but I installed the Scripter plugin and wrote a quick script to apply auto layout to multiple selected frames at once without grouping them. I don’t know much programming but this is what I wrote, and you can just change it to VERTICAL if that’s what you want. It seems to work, though probably not the answer you were looking for.

for (let n of selection()) {

	if (isFrame(n)) {

		n.layoutMode = "HORIZONTAL"

	}

This plugin can do it:

Thank you Gleb and Brian!