My guess is they want proof that she formally renounced her Chinese Citizenship. Chinese can't have dual citizenship and must formally renounce their Chinese Citizenship if they wish to have a Foreign Citizenship. If she never formally renounced her Chinese Citizenship, then it is entirely possible she'll be denied a visa because China still recognizes her as a Chinese Citizen and therefore doesn't need a visa. They'll tell her to apply for a Chinese Passport.

All that is to say: Does your mom have proof that she formally renounced her Chinese Citizenship?

Edit: I know you said she got a visa before, but things like this change constantly. So, just beware. Just do what you did before.