This is a small note, but IIRC that instruction on the z80 is actually undocumented. It's easy to tell what it should be though since all the shifts are the sam

This is a small note, but IIRC that instruction on the z80 is actually undocumented. It's easy to tell what it should be though since all the shifts are the same opcode except for two bits representing the direction and type (arithmetic/logical). But that would be why the behavior is so weird - it's just whatever happens in the CPU when both those settings happen to be set.