Unfortunately, midi standards are tossed out the window with iOS apps. On the plus side, I’ve found that most iOS developers are pretty good at answering emails and addressing problems with their apps. So if all else fails, contact the dev directly and if it is a bug we’ll probably see an update in a week or two. Let us know how you make out.
Edit: Is this happening without Audiobus as well? Chordion has a similar issue where midi out won’t work with some apps when Audiobus is turned on.