Performance Tile Filter not producing correct results

Summary:

I am trying to produce a measure for number of maternity cases currently in progress where the visa permit type has not been entered (is null).

It produces results but is ignoring the filter where the visa permit type is null and producing all cases in progress.

This is across two subject areas

"Workforce Management - Absence Real Time"

"Workforce Management - Person Real Time"

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Can someone please advise where I am going wrong or how can I get the correct results?

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