First op, I want to send hugs to you, this seems to be very important to you.
I'm going to throw some questions back at you, not to pick on you but to honestly get you thinking about some other views.
Now here are my random thoughts on this and the topic comes up now and again.
We all have these "leave it to beaver" type expectations of how grandparents are supposed to act, then when the grand parent doesn't do it we get mad. Why? Grandparents are the ones who decided for whatever reason how much involvement they will have in their grandkids lives. Yes, in a perfect world all families would be like the Huxetables. Life is rarely like that.
My MIL is Jehovah Witness so she never gives gifts or birthday cards so kids can and do survive without them. Be that as it may, I don't worry what other people do for my kids, dh and I make sure they have the type of birthdays we want them to have. I doubt if my kids really noticed until they were young adults if Mom-mom gave them a gift. I've read stories about people going to disney world with the grand parents than getting mad because GP's didn't want to babysit the kids.
In general, I don't think people "suddenly" begin acting a certain way, I think they give little signals along the way. Maybe you just didn't want to acknowledge them. (this is just my pea brain pyscho bable) What type of grandma is she the rest of the year? If she is actively involved in their lives then forgetting a birthday is not the end of the world. If she doesn't pay much attention to them the other 300+ days, then that's not going to change.
Did I read you say she doesn't acknowledge your birthday because you're not family? Sorry IMO that was a big clue. Evidently she's one of those 'bloodline" type people. I actually had a cousin like this, only direct line"Sanchezes" counted in her world? go figure.
Lastly, I also believe in what the great poet Laurete Mya Anglo said, "when people show you who they are, believe them". You say your mother finds facebook more important then her grandkids? Ok, there's your fact. Stop expecting her to come through like Maria Barrone. she will only keep disappointing you.