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This indeed looks like a bug.
As a workaround, test whether the graph has loops before computing the chromatic polynomial.
sage: G = Graph([[1, 1]], multiedges=True, loops=True)
sage: G.has_loops()
True
Fixing the bug should just amount to special-casing graphs with loops (testing for loops as above) and including a doctest (to prevent the bug from reappearing).
