Relevant settings: There are no restrictions on the camera app and the "Scan QR Code" setting is turned ON
After more research, the following has been identified:
After un-enrolling the device with MDM, the QR Code scanner works as expected.
A factory reset selecting "Erase All Content" option was completed on an iPhone6S and an iPhone7 (iOS 14.0.1) and this resolved the issue.
After deploying an empty MDM policy to a test phone, the functionality was still not restored.