Pressing return from a heading highlight next line

Sometimes if I press return after specifying that a line is a header, then the next line get highlighted. In this case the next line is empty. So there isn’t anything to highlight. No text from this file was ever highlighted before so I don’t know where it’s coming from. I keep having to undo all highlights and I almost never use this feature.
I’m wondering if it’s a bug or a feature? How do I turn this off? I