Dont know if this is intentional or it fell between the cracks somewhere, but if you use the screen brush or surface brush to groom the hair you are no longer able to adjust the density of the guides.
This means you have to groom with the amount of hair you will be needing to create the final look.

Hi,

if you hit the ‘recache strokes’ button at the top of the Guide Groom parameter tab that should update your groom.

Wow, it works like a charm.
These grooming tools are brilliant!
Love the fact that everything is self contained in the file so you dont have to use maps, but also have the ability to do so if you wish.