Report Leaving Assigned Altitude?

And I didn't say that you said it would.

Ok, as long as we're clear on that. ;-)

Let me rephrase the question, how could the pilot's behavior, that
is, not reporting leaving a previously assigned altitude on a
discretionary descent, possibly displease ATC?

The only possible thing that I could think of is that if ATC were
waiting to assign that altitude to another aircraft. However, I would
think that ATC would not assign a discretionary descent if that were
the case.

So, the net result is that I can't think of one.