I have now had reports of the same behaviour from 2 users so the issue is not confined to my device.
Re-installation of the package in release or debug has had no effect on this behaviour and no errors are evident in normal or unfiltered logs.
Presuming an issue with Google I deleted the app from the list of authorised package names in the API console, I saved the data then closed. I then recreated the exact same package name with the same SHA-1 signing-certificate fingerprint and saved.
After a few minutes the app now works normally again without the blank map.
How very frustrating