I doubt it.
I’m pretty sure each of my kids think that they are their Grandmother’s favorite. That’s the beauty of being a grandparent. We don’t have to worry about any of the other stuff. You know, the actual parenting. We’re all about the attention. And, speaking from my somewhat limited experience, there seems to be plenty to go around. We’re like bottomless pits of praise and plaudits. An endless fount of fussing and fawning. No grandchild goes attention-less.
So I say no -- Middle Grandchild Syndrome is not a thing.
Of course, I could be wrong. If I am, you can be sure I’ll let you know.
GRANDCHILD #3 UPDATE: