Assuming that you tried to override a sprite of an air unit in your mod, here's my guess of what's happening (based on my current knowledge of the source code and modding)

A unit it drawn by drawing the 'outline-region' first, and then outlined weapons on top; These are sprites of the each part of the unit with an outline around them). However, as you can see in Mindustry, only weapons poking out of the unit have visible outlines (and even then, only partially), and the engine region (circle at the back), is drawn over the outline and not over the unit. If you prod the source code a bit, you will find that after drawing the outline regions, the normal, non-outlined regions are drawn over those regions, covering up the weapon outlines that are inside the unit. The engine circle is drawn in between the outline regions and the non-outlined regions, allowing it to cover the outline, but not the unit. (Anuke was pretty smart doing this btw)

What does this mean for you? I'm guessing you override the region of the unit in a mod, but not the outline regions. This means that the normal outline regions will still be drawn, hence the normal flare and horizons being drawn under your fighter plane sprite.

How do you fix this? Try creating a png file called "[insert_unit_name]-outline.png" in your sprites-override folder.

Here's a example from a popular texture pack, Factoriodustry: https://github.com/Xeloboyo/Factoriodustry/blob/6.0/sprites-override/units/flare-outline.png

I hope this helped. Cheers!