OK, I just found out from further reading that there is a work-around to the problem I reported (above).

As an alternative, I can use the Audio tab and select 'Render Song To Audio File (WAV, AIFF, M4A)'.
This will successfully write an M4A file to disk.

It does seem that the files are perhaps larger in size (perhaps 2x). Not sure why, but maybe the default sampling/bit rate is different.

Regards, TS