If you are talking about page 3, then I see no problem, the box has a character ( as running header because that is the first character that has None character style applied to it. See my screen recording showing the styles applied on that page. For other pages like 5 onwards it's picking the text from the anchored box which I am not sure if it is a documented behavior
I have chosen the "running header character style" to appear as the running header.
I am not sure why it is choosing another character style.
I've just created another text variable which seems to have solved the problem. Again i don't really know why it didn't do it with the built in text variable.
Thank you for your help.
The inbuilt running header you used initially that I looked at has been defined to look for [None] character style. You can see its definition by clicking the edit button in the dialog that you used to create the new running header variable. So it's working as it has been defined
Don't use character style "None" for the running header.
Instead, create one distinct character style and give it a name that won't confuse you.
After that assign this style only to the text you want to appear in the running header