Question: piecewise, simplification and 'is'

Why Maple 11 does not reply by true for the following snipped:

assume(m::real):
PIECEWISE([exp(m)-exp(-m), m <= 0],[0, 0 < m]) = min(0, exp(m)-exp(-m));
convert(%,piecewise,m);
is(%);

It handles the 3 cases 0<m, m=0, 0<m correctly. I find that strange.
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