Double spacing bug in 'running header' feature.
Pretty sure the problem is that the running header feature is seeing the soft return (shift-enter) as a spacing. But the spacing bug only happens on the page, it's taking the information from.
The following pages don't have this problem and the only difference is that these pages take the 'running header' information from another page than their own.

Can see this problem dates back before 2016 and as far I can see, hasn't been solved.
Adobe employees are also confirming it's a bug in the posts.
The last post in this thread by jutaipeter actually fixes the problem. But it's a workaround:

