The running head is designed to be a fixed height, regardless of screen size, hence no wrapping. You could write your own CSS to implement line-wrapping, but I don’t recommend it.
Instead, you might consider …
- Design the running head for desktop, by which I mean to keep what you already have, as “it looks great on desktop”.
- Under Running Head => Title, set:
- Breakpoint: at 640px
- Breakpoint Visibility: Above the breakpoint
- Under Header, turn on Enable "Header, Secondary.
- Under Header => Header, Secondary, set:
- Appears at breakpoint: at 640px
- Customize further to your liking …
And this will hide the title from the running head on smaller displays, and display it instead in a mobile-only header.