Assuming a PC grabs a flying monster..

[Aulirophile]

Right... and I'm saying that you are most clearly holding onto a flying enemy.

All the more incentive for them to escape grab.

Well, as I said, if you want to categorize a monster as a surface, sure, that is how it works and the monster is immobilized in it's current square.

 

[Amaroq]

I'd make something different up if it comes up in my home game.

This.

It seriously feels like RAW started with a 2-D miniatures wargame mechanic, onto which "flying", and 3-D in general, were just tacked on. (Rather than built in from the get-go.) Admittedly its very tough to represent 3-D in 2-D, but I don't think the RAW does it very well.

So, I'd recommend that, in general, folks rely on their DM's adjudication in any flying encounters rather than trying to understand the RAW with precision.

Just tell your DM what you want to do, and let her worry about whether it works or not!