I just fixed it (see solution below) – so maybe this is interesting to some. Since I don't know if this is an actual bug or a feature™, I post it anyway.
I really appreciate the new feature running headers field. Unfortunately, I'm getting a bit confused here. In my scenario, I have a chapter number and a chapter title and want to present it on every page. The format I have in the text block on the master page is the following: <Running Header>—<Running Header> but for some reason, I have a line break after the first one which should resemble the chapter number.
I inspected the characters and found that a invisible character is added after the chapter number.
<0x00> // which resembles a null symbol according to unicode
I also inspected the source field but found no special character after the chapters number.
––––
Solution, kind of.
In my scenario I created the chapter numbers style as a character style instead of creating a paragraph style like I did for the title.
Easy fix was to just use a paragraph style for the chapter number too.