Hard to say what you are doing wrong, because it works fine for me.
After adding the space, are you pressing both the save plugboard button and the apply button at the top left?
Is the space gone from the template in the "existing plugboards" box after you press the save plugboard button? What I see after pressing the button is
Code:
any format:any device = ([{title}{series:| (|}{series_index:0>2s| |)}] -> title)
The space is there.
Is more than one plugboard defined? It could be that a more specific plugboard without the space is being used in lieu of a more general one that you are changing.
If for some reason you can't make it work correctly (gremlins), put the space between {title} and {series}. It will be removed if there is no series, because leading and trailing spaces are always stripped after the template is executed.