https://wiki.nesdev.com/w/index.php/Bandai_FCG_board states that "The IRQ counter is clocked by M2. There is no reload value, nor does it stop: after it becomes 0, it becomes 65535. Writes are not shadowed, so you either need to be careful about timing or pause the counter while you manually reload."
This cannot be true for Famicom Jump II (Mapper 153), as the screen split at the title screen will be glitched, and CHR RAM will become corrupted, if writes are not shadowed. FCEUX loads the IRQ counter from the shadow value when register 0xA is written to, and has registersx 0xB and 0xC go to the shadow value instead of the IRQ counter itself. When I do that as well, the problems disappear.
This cannot be true for Famicom Jump II (Mapper 153), as the screen split at the title screen will be glitched, and CHR RAM will become corrupted, if writes are not shadowed. FCEUX loads the IRQ counter from the shadow value when register 0xA is written to, and has registersx 0xB and 0xC go to the shadow value instead of the IRQ counter itself. When I do that as well, the problems disappear.