I've been playing with your dashboard and am having trouble replicating the issue you are having. It also took me a while to figure out where the Customer filter was coming from, but if you take a look at your 'hidden' field this might help explain the logic.
The green circled areas work as a filter when 'only relevant values' is selected in the drop down caret for the filter - this means that the red and green areas are filtering the customer filter. This selection persists even if you hit the 'all' sheet, thus for example the selected red + all = 3 customers which will be in the filter. If you unselect red, then all 8 customers appear.
In this situation, the All sheet is doing its job, except that all 8 customers on the view is the same as red + green being unselected so you don't actually see any difference.
You can see it in action in my version of your workbook where I have included the relevant values so that you can see them in your hidden sheet where the filter was being driven off.
HTH 18.2 attached
sample.twbx 23.2 KB
Is there a way to get the full list when I click on "All"?