3813291 - Why user could submit expense report and leave one mandatory custom field blank?

Our company has one custom field added to expense entry form which applies to one specific expense type. Also it was set mandatory.
However, we found one entry in user's report leave this field Blank and user submitted successfully. As we known, for mandatory field, user cannot submit report until fill in otherwise one red exception "Missing Required Field XXX" will trigger.
May I know why this happened?

Why user submitted one expense report and leave one mandatory custom field blank , KBA , CON-EXP-ADM-FOF , Forms and Fields , Problem