Index is a table calculation and table calculations are very sensitive to the dimensions that are on your view. To fix this, you'll need to make some modifications to the way the Index and Top N Filter table calculations compute. Right-click on the Index pill (on the rows shelf) and choose "Edit Table Calculation". Set it up to look like this. Make sure that Customer Name is first in the list (if it's not, drag it up).
Then do the exact same thing on the Top N filter pill, setting it up to look the same way.
See the attached workbook.