Rankx with ALLSELECT +ALLEXCEPT

hi, Anonymous 

RANKX (
ALLSELECTED ( CUSTOMERS[CUSTOMER_PARENT_ID] ),
CALCULATE (
[Total AR Amount],
ALLEXCEPT ( CUSTOMERS, CUSTOMERS[CUSTOMER_PARENT_ID] )
),
,
DESC,
DENSE
)

here you use allexcept as calculatemodifier so it remove only filter context not ignoring outer filter value 

for ignoring outer filter value you will have to use it in table function not in calculate modifier.