I'm pretty confident the basin waste is the lower of the 2 going into the stack, OP suggests it runs under the floor for some part of its journey to the stack, so makes sense, if bath is against the wall, then bath waste could well be higher.
Basin waste may not be all the way across the stack, but if it's sticking out too far into the stack, crossflow could have partially blocked it inside the stack. I'd be interested to know what the basin flow would be like with the pipe removed from the boss adaptor.