I do think it's in the spectrum of totally normal behavior. Not sure how longs it's been going on but you might find it passes very quickly.
My boys would do strange things like that for a couple of days, and then once they were bored with it, or figured out all they could figure out by say, rocking back and forth in the highchair, they would be done and on to the next discovery.
You could try tying a sash around his middle to hold him a bit more secure. Depending on how much time you have to fart around with meals, you could just get him down and try again in 10-20-30 minutes or so. Maybe he's just not hungry enough to focus on the food.
Your babies are too cute! Love the avatar!