For what is worth, this is one of the very few reasons why I still need to change the Flow editor to edit source.
Copypasting a header from i.e. a wiki page into a Flow topic will result in a string inheriting the header format. If you want to remove that format (i.e. because you are only referring to a wiki page inside a paragraph, a very frequent use case), then the only way to do that is to change editing mode to 'edit source'.
Similarly, sometimes you end up with a bullet list that you want to divide and (as far as I'm aware), the only way to do that is to do to 'edit source'.
This type of problem also happens in VisualEditor, GMail, and surely others, but they are equipped with a "remove format" button that solves this problem. I know you have been very selective choosing buttons for the minimalistic Flow toolbar, but maybe this is a button to be added? The ultimate goal should be that users don't need to rely on 'edit source' mode except for very specialized tasks.