Audible stops when iPhone locks
Today, 2/21/22 Audible advised me that this is a known problem with a fix TBD. The kluge work-around is to change in Setting>Display & Brightness>Auto-Lock to Never. Obviously this is sub-optimal as this setting will promote battery drain, and we can only hope Audible will issue a fix, which probably also means deleting the app, then re-loading it and all the titles you'd like to have. Fingers crossed.
iPhone 12