It usually is the smaller ships that tend to attract the more interesting of creators that like to keep things in character in my experience.
Plus, there's no clear seme/uke 'tropes' attached to them like with the others. Gladnis? Immediately they assign 'seme' to Gladio (bigger, muscular, and macho) and 'uke' to Ignis (somewhat shorter, leaner, domestic, and cooks) and play it painfully straight. Because big manly men never want to bottom, of course. Not in fujoland, anyway.
Promptis? As you highlighted in your rant, Prompto almost NEVER tops in 90% of the stuff he's in and being with Noctis does little to change this.
Now, I won't say there aren't some that try to do this with Ignoct, but not as often from what I've seen.
As for why that artist suddenly fell into the trend? Methinks they're probably going through a rough cycle and is projecting their angst onto Prompto at the moment. That's my guess, anyway.
And thanks for the extra Ignoct art.