I had another look at this and found that launching iReal Pro first (making sure GB was not already open) then opening Garageband worked.
I tested Play in iReal Pro when I first launched it, when GB switches to iReal Pro as it connects, and after pressing Record in GB. There is a little iReal Pro icon in GB you can tap to switch to it. Also as iReal Pro was playing I could tap for the red GB recording bar (at the top) and tap it to switch back to GB as it was recording.

I also turned off metronome and count-in within GB.
I hope this is useful, Daniel. This should work with your iPhone. When you get to it, let me know if you get it working too.

For anyone else reading this in the future, Apple may change their implementation of how this all works. For now, it seems to work as described.