Heartgold confoundingly only produces corrupted saves.
So when I save this game in Delta, it always says “Saving a lot of data” even if I save twice in a row. Then when I go to load the save, it gives me the above error. This happens even on the first save of a new game, with no save stating done prior. I’ve tried Heartgold roms from two different sources, and meanwhile Pokemon Black and Ace Attorney don’t have this problem.
I tried loading the game up in Manic Emu and had the same problem. But Manic has two DS cores! I swapped from MelonDS to DeSmuME, and the corrupted file worked like a charm! Unfortunately in DeSmuME there are some graphical glitches and the game slows down randomly, which I have to keep fixing by pausing Manic and unpausing it. So I saved (and there was no “a lot of data” message this time) and tried opening it in MelonDS, still corrupted. I don’t know what’s going on here.