I was able to repro one issue that could have been contributing to your broken experience - there's a slight delay between, for example, clicking text in a heading and having the "#" markdown decoration appear. This is to prevent the mouse location from shifting mid-click and causing text to be selected unintentionally (obsidian does this too). But there was a bug that was causing a cascading set of failures if edits happened during that delay window, which is likely what folks who are clicking around at random points in the editor and adding text are doing. I fixed it in 0.4.2, which should be live now.