HerHusband wrote:

"When I removed the plates I found that the electrical boxes were not
attached to the studs"

I got the impression the box was just "floating" somehow other than the
front strap for the fixture mounting. I couldn't see a mounting bracket in
the photo, but maybe it's there in the bottom of the hole?

Either way, as long as the box is secured somehow, you should be fine.

Retrofit boxes use wings that fold out behind the drywall and are not attached
to any stud.