Thank you for talking about a real use case :). The m. causing a new workflow issue to be introduced is exactly why they decided "m." should not be treated as a trivial subdomain when they rolled it out, it was only that way during testing.

As you said www.example.com being functionally different from example.com was already a broken workflow, users weren't differentiating it. Continuing to display something users haven't understood for 20 years was not considered a strong enough reason to simplify the URL display now in a way that causes the same errors users were commonly making anyways.