In iOS 11.4 there were many changes to the audio settings by Apple. In order to fix the AirPlay compatibility issues with iReal Pro, we had to change it on our end as well so currently it doesn't work with Audiobus 3.
You should still be able to use your instruments and effects while iReal Pro plays in the background or in split screen.

If you record with GarageBand or similar apps, export your iReal Pro backing tracks in WAV format which is now a very fast process.
Then select 'Audio Copy' from the share sheet options and paste it into GarageBand.