There might be something different about Boss's MIDI implementation compared to other MIDI devices that other apps are able to handle and my app isn't. I'm actually surprised any app can connect, because the Boss doesn't appear to have a class-compliant interface; the user manual says a driver is required for USB MIDI, which typically means it doesn't work with mobile devices.
Now that Android has some built-in MIDI capabilities (in Android 6 and above), I'm planning to rewrite the MIDI connectivity in my app and that might help. Meanwhile, I'd suggest adding a USB MIDI interface to your setup, but it looks like the Boss doesn't have a 5-pin MIDI port, and I'm not aware of a MIDI interface that has a USB output.