What looks like a vole-cut to me, and the plant was right next to a burrow and a feeding station.
It's clear, especially when you're doing The Big Butterfly Count, how much insects love ragwort. But I thought mammals avoided eating the fresh plant because it tasted nasty. However, this does look like water vole-gnawing to me (third photo down) as it was high up the stem, a slanted cut, and right next to a burrow and other feeding. I've never found ragwort in feed stations, so my guess is a water vole just gave this a try, thought it was horrible and gave up.