since i have no iPhone to try this with myPhonak app, i did a search here in the forum and came across this tread
This tread were a discussion after the release of MyPhonak app version 6.
Ouote: " if you have the ’sounds and haptics’ switched on in your settings (that is what makes the sounds in your phone such as keyboard clicks, this setting is also responsible for the ‘lock sound’ you hear when you turn your phone off) they have found that when an iPhone makes the ‘lock; sound, it puts the iPhone into streaming mode and it reverts back to Autosense - so if this is happening they suggest turning off ’sounds and haptics’ in your settings. This way you can ensure that your hearing aids ’stick’ in the programme you have chosen. "
I don’t know if this issues are still relevant for iPhone , but here this were mentioned