Thank you @Levon for your assistance. I now have it working!
(recording here in case anyone else hits this issue)
Going through all the documentation the solution I found was not obvious. (well not to me) The key to get it working for me was to update the proguard-rules.pro file to include:
-dontobfuscate
in addition to the data specified here:
It is mentioned in the next section in that document - but not as a required change.