Buggy code is here: https://github.com/mgba-emu/mgba/blob/694b80d289cdd1eadf47223bc2d4f2bbef0393e6/src/gba/core.c#L703 MB is held in gba->mbVf rather than gba->romVf, so gba->romVf will just be null.
Buggy code is here:
mgba/src/gba/core.c
Line 703 in 694b80d
MB is held in gba->mbVf rather than gba->romVf, so gba->romVf will just be null.