Eggy, it might very well be the actual device? I use the Yammie and have had no issues with either my iPhone or iPad.
Re the MIDI question, yes certainly…that is how I have it set. It sends a message to my synth (PC4) which then selects the multi I am using, and it also sends the data to the BB. I use two Yammie MD-BT01’s .
I also setup (when needed) MIDI “buttons” on the chart page which I can also set to any command so while “in song” I can send different things to the BB, EG send a volume change for a quiet section, or a drum kit change, or even a tempo change - of course you need a “pair” of buttons if you need to return to the main song again.
Onsong can do most of this stuff too, I just find the interface too cluttered, I do not like swiping up and down and it jsut seems too “busy” overall. But, the stuff I describe here can be done in it as well, WHICH gets me back to the earlier suggestion that perhaps it is the device??