field validation not running

Summary:

The required field seems is missing validation

Content (required):

As the below indicates, the Causal Part is a required field, same as the rest of the three, but when the WO is submitted with no info entered, the validation in this field seems is not running but the rest of the three which have the same Visibility set up are showing error.

Can anyone advise where I did wrong ?

Screen configuration comparison

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