Bug with matrice products over Symbolic Ring with modular integers

[tmonteil]

As reported on ​this ask question

sage: sage: var('A,B,C,D')
(A, B, C, D)
sage: S = matrix(SR,2,[[1+3*A,3*B],[3*C,1+3*D]])
sage: R = Zmod(9)
sage: g = matrix(R,2,[[2,1],[2,6]])
sage: gi = matrix(R,2,[[6,8],[7,2]])
sage: g*S*gi
[        6*B + 3*D + 1 3*A + 3*B + 6*C + 6*D]
[            0*A + 0*D         3*A + 0*D + 1]

While we have (look at the second line):

sage: R.<A,B,C,D> = PolynomialRing(Zmod(9),4)
sage: g = matrix(R,2,[[2,1],[2,6]])
sage: gi = matrix(R,2,[[6,8],[7,2]])
sage: S = matrix(R,2,[[1+3*A,3*B],[3*C,1+3*D]])
sage: g*S*gi
[        6*B + 3*D + 1 3*A + 3*B + 6*C + 6*D]
[                  6*B         3*A + 3*B + 1]