devkitPro

Grit sometimes produces corrupted output. To recreate, consider the bigmap demo from TONC. Open kakariko.png in mspaint (version 11.2304.33.0) and then save without editing - this will change the PNG chunks in a way that Grit doesn't seem to know how to deal with but should otherwise leave the image data the same. Then make clean, make again, and launch bigmap.gba in an emu. Observe that the background is now corrupted (but still recognizable).

As I said on the github issue

mspaint isn't a suitable tool to work with image data for GBA graphics, you need something with much more control over palettes and image formats. Grit can't work miracles.

We'd recommend something like Pro Motion NG from https://www.cosmigo.com/ or Graphics Gale from https://graphicsgale.com/us/