The instances may have different versions of the Lemmy software, which may be causing the inconsistencies.
I use the feeds Open RSS provides for Lemmy and looks like it includes images. If it doesnt, just hit them up and ask if they can implement. They're pretty chill and resolve RSS feed issue pretty quickly.
http://openrss.org/lemmy.ml/
You can keep bringing it up. But, in my exp, if you dont have anyone high up that can support your perspective, it's just gonna be an uphill battle. And is likely just going to make the other devs unhappy with you.
It's unfortunate but most devs dont really like anything that's going to cause them more work (e.g. more code reviews, higher quality changes, looking at the bigger impact of their changes etc).
If you don't have someone higher up —maybe the manager of the managers of those problematic engineers—I'd just make more tests around the areas that are breaking and require those tests to pass before merging code changes. Devs may not like to work harder, but they damn sure dont like seeing a bunch of red X's when they open a PR lol 😃