iPhone 16, iOS 18.5 - Vocal Shortcuts keeps quitting, will not stay selected

I have a new iPhone 16 Pro (June/2025) and when I select Vocal Shortcuts ‘on,’ it only remains active, or ‘on,’ for short periods of time. It disengages or quits and I need to keep going in to Accessibility to reactivate it. The orange dot shows in the Dynamic Island when I select Vocal Shortcuts on, then (quite randomly) disappears. Sometimes it’s active for several hours, other times less than a hour. Tried restarting iPhone with it deselected, then selecting ‘on’ again (and other attempted fixes) but it keeps quitting. Also, just updated to iOS 18.5 but this has not corrected the problem. Had the same problem with iOS 18.4.1 before. Any suggestions on how to fix the repeated quitting of this useful feature would be v. much appreciated. Thank you.