When I switch to another synth, or an app that plays video like YouTube, and back to Audulus the audio stops working. Also, clocks stop responding so it’s like the audio engine crashes. Exiting and reloading the document doesn’t work but killing Audulus and restarting the app brings everything back to normal.
I’m on a 2017 iPad Pro 10.5" with iOS 12.1.1. I have version 3.5.3 of Audulus that I purchased a few days ago.
EDIT: I tested this on my iPhone 8 and the same thing happens.