Running feet on two lines on the first chapter page

We display the chapter title (at least the short title) in the feet on the right page (PDF export).

We notice that when exporting the book, the title, even if quite short, is displayed on 2 lines on the first page of each chapter.

From the second right page, it is displayed as expected (short title + page number on 1 line):

Other example:
First chapter page, on 2 lines:

Expected display from the 3rd page and ff:

We use theme McLuhan, Version: 2.13.0.

I haven’t found what I should change in the CSS to always display running feet on one line.

Thanks for any advice you may have!