Thanks for responding, Matt.
I tried removing and reinstalling the app - a couple of weeks ago, and again today - and the problem persists.
I've noticed that I have the app in my 'external' SD card. At some point I must have moved it to release space in internal storage, and I'm wondering if that's the cause of the problem. I'll move it back to internal storage to see if that makes a difference, and I'll report back.