It did back then, your mother most likely lost her British subject status when her father (your grandfather) obtained US citizenship. Multiple citizenship was recognized by Canada by the 1977 Act, but it did not retroactively return citizenship to those that had lost it by naturalizing in another country, as directed in the 1947 Act; those people became known as "Lost Canadians". This was remedied first in the 2009 Act, which restored citizenship to those that had lost it because of the 1947 Act. In the case of your mother, her citizenship was granted by the 2015 Act under
3(1)(k), which benefited British subjects that lost their status before the 1947 Act; as a result of this, your claim to citizenship by descent would be because of 3(1)(g).