On Tuesday, September 29, 2015 at 1:50:18 PM UTC-4, Arthur Conan Doyle wrote:
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.

In the situation that started this thread, by having the junction box
attached to the mounting plate and then having the mounting plate screwed
to a stud, the fixture is much more secure than it would be if the junction
box was held in by a couple of tabs.

The mounting plate is pretty thick and the fixtures have some heft. The
torque on the tabs would be significant.