Heroes 5 – by Azmadon (Izmail).

Perhaps the issue lies in the initial expansion pack I'm playing. That is, playing in fullscreen.
I believe this aligns more with the concept of "Normal."

Yes, that's what I meant. It's an old game, and I remember that some adjustments were needed to play it on a widescreen.
Take a look, but I would try to fix it if possible, as it will improve the modding result. Otherwise, your icons should be square, but during testing, you see them as rectangles, which means they don't look the way your users will see them.
But overall, good job, and good luck.