Based on the current documentation, my understanding is that Alert No Answer is incremented whenever an agent is alerted for a conversation but no answer event occurs. The metric definition doesn't appear to distinguish between ACD and Non-ACD conversations.
One important point is that Alert No Answer is different from the Not Responding routing status. An agent is only moved to Not Responding for ACD-routed conversations after the configured alert timeout, but that routing status is separate from how the Alert No Answer metric is defined.
Based on your tests, if Non-ACD conversations are still generating an Alert but are no longer incrementing Alert No Answer, that does seem different from what the documentation describes. I don't recall seeing an announcement about a change to this behavior, so it may be worth opening a Customer Care case to determine whether this is an intentional product change or an issue with the metric calculation.
I'm also curious if anyone else in the community has observed the same behavior.