I've created a project template here, and linked it from the readme in this repository:

https://github.com/evanbowman/bpcore-project-template/tree/master

I will keep discussing here also if there is a repo as it is already opened.
So I executed the build and generated NewProject.gba but mgba crashes with a segmentation fault.

Also the build didn't output anything like an error or so.

I will share to you on the gbadev discord the gba rom if it is useful to debug.

I was thinking to create a bash script to execute before build.lua that fix the colors of all the bitmaps.

convert background.bmp -type truecolor background.bmp
convert tiles/test.bmp -type truecolor tiles/test.bmp
output=$(./build.lua 2>&1)
if [[ -z "$output" ]]; then
    /usr/games/mgba-qt ./NewProject.gba
else
    echo $output
fi

Also the rom after build still keep the name BPCoreEngine and not the one defined in the manifest.