Unable to check Null value of numeric EFF in item rule

Summary:
We are trying to populate the value in a (Destination) EFF based on the value in another Number type (Source) EFF. The rule works if the value in source EFF is not null. If the value in Source EFF is cleared the rule doesn't work. We tried multiple functions like CHANGED, ISNULL, CONTAINS, + (Concatenation), LENGTH (= 0) but nothing is able to capture the event when the source field is cleared.

Populating the source field from a blank EFF to a not null value triggers the rule, Switching value in Source EFF from one not null value to another also triggers the rule. ONLY when the source field having a not null value is cleared to be blank / NULL then rule doesn't work.