SharePoint List Validation works differently based on user

My organization uses a SharePoint List to enter, edit and store certain data.

There are two users of this list for this issue. Both users are Site Owners. 

User 1

User 2

One column in the list for Member MBI Data has the below validation. This is a single line of text field that allows numbers and letters. 

=OR(ISBLANK(MEMBERMBI),LEN(MEMBERMBI)=11)

Error message:

'Member MBI must be 11 characters.'

This validation forces the field to have either 11 characters OR be NULL. A NULL value is allowed. 

This validation is set in Column Settings > Column Validation. It also shows the same code in List Settings > Edit Column > Column Validation.

The issue: 

User 1 can Create or Edit and save any List item with a NULL MBI and save, OR can even remove the MBI data so the field is NULL and save (This is expected and working properly).

However, if user 2  Creates a List Item with a NULL MBI and saves, the save will be disallowed / show the validation and should not.

Error message:

'Member MBI must be 11 characters.'

OR 

If user 2 Edits a List Item and removes the MBI Data so the field is NULL and saves, the save will be disallowed.

Error message changes to this:

'Error: The server was unable to save the form at this time. Please try again.'

I tried removing user 2 completely from the SharePoint List and adding back both as owner and separately, as member. The issue replicates in both cases. It seems like a permissions issue for user 2 but both users are set as Site owners. 

Any idea what would cause a List Validation to force NULL on one or more user accounts but not others accounts even while the validation is coded to allow NULL?