We had a few issues with MIL pushing her partner to be called Grandpa. DP and I were furious. She's do it behind our backs and DS1 would just go along with her calling him Grandpa. We don't see them very often so it doesn't really happen much. If it did then I'd be a whole other story and we'd most likely have to say something.
We just explained to DS1 that we call MIL's partner by his name and that it's easier, etc.

I'm sorry I'm not much help. Just wanted to share what happened with us.
GL and

xox