Okay I had this exact problem but slightly different.
My husbands family are from a different culture however in that culture it is not cultural to call grandmother "mama"
My MIL tried saying it was cultural that's why her daughters children call her "mama" my husband repeated and told his mother no it's not and to stop lying as he didn't cal his grandma this bla bla bla.
It actually turned out, because she provided childcare for her daughters first born. She forced the boy to call her mama and then he called his own mum by "mama (SIL name)" then the other children followed.
I was having none of it. She would try say to my daughter (come to mama, mama apple93 will get you later) luckily my dh backed me completely and it resulted in very low contact NO ALONE TIME and lockdown happened which helped as dd started calling me mama in lockdown.
Now the reason I was so against it was because it was definitely a power play in my situation and still now she says she loves sil children more than mine because they call her mama 

In your situation it does seem to be a tradition and your husband called his grandmother this which I think softens it.
But I get it. I spent a year obsessed on this issue it really put a dampener on my child calling me mama.
Could you perhaps compromise and call her mama (name) as tbh your child will call you mama only for a short time then it will turn to mummy most likely
Please don't let this upset you to the extent it did to me