mGB does not prevent writes to VRAM during LCD Mode 3. (For some reason, OAM is properly locked out.)

gb/memory.c:242 return gb->video.vramBank[address & (GB_SIZE_VRAM_BANK0 - 1)];

This should be appended to

if(gb->video.mode == 3) {
    return 0xFF;
}

This needs to also be applied to lines 312-313 (blocking VRAM writes), and to gb/io.c, lines 491/492 and 500/501, peppering

if(gb->video.mode == 3) {
    return;
}

(This blocks GBC palettes during Mode 3)

Example ROM, which exhibits this behavior: https://github.com/eevee/anise-cheezball-rising

Note that even with this applied (on my local fork), the output doesn't match BGB's. Implies timing errors, maybe tested by gekkio's ROMs. But that's a different problem.