deep copy not working in _match_dims ?

The _match_dims(poly1, poly2, copy=None) in canonical_polynomial.py doesnt seem to be doing deep copy of the polynomial correctly. It is altering the multi index of the input polynomial.

Minimal code to reproduce the issue:

mi1 = mp.MultiIndex.from_degree(3, 2, 2.0)
coeffs1 = np.array([1, 2, 0, 4, 3, 0, 5, 0, 0, 0, 0])

mi2 = mp.MultiIndex.from_degree(2, 2, 2.0)
mi2_exponents = mi2.exponents.copy()
coeffs2 = np.array([1, 2, 0, 3, 4, 0])

can_poly_1 = mp.CanonicalPolynomial(coeffs1, mi1)
can_poly_2 = mp.CanonicalPolynomial(coeffs2, mi2)

res = can_poly_1 + can_poly_2

assert can_poly_2.multi_index.exponents == mi2_exponents

Expected behavior: the input polynomials and their multi index set remain unaffected after addition.