Is this common?

It's a poor attempt at filling the exhaust crossover as you've already determined. Typically, it's done by melting aluminum until it's liquid, then pouring it into the port. When it cools, you grind it away to match the other ports. The reason for doing it is to eliminate the turbulence created by the open hole in the back of those 2 ports. Even if you block them with a gasket or sheetmetal at the intake, you'll get turbulence in those 2 ports. It's a nice trick if your car doesn't get any winter use.

I've done this a couple of times using old pistons and/or scrap aluminum.