I have the plastic ones too. One of my pigs (I have a herd of 10) loves to flip them over as soon as we set them down. So we have started just laying the pellets in piles on the fleece (since that's where they end up anyway lol). Maybe we will reintroduce the bowls later on to see if she will still tip them over or not.
I use ceramic ones. It's quite funny because they can't tip them over but they will grip the edges with their teeth and drag them along the fleece. One of my pigs will drag it to the outside of the pigloo so she can stay inside it and eat!