Dropdown - filtering values conflicts with preset values

Dropdown - filtering values conflicts with preset values

I have a form with a CUSTOMER dropdown lookup field and a SITE dropdown lookup field. The SITE lookup shows the SITE ID, SITE NAME and SITE ADDRESS.
The SITE field has criteria 
CUSTOMER_ID == input.CUSTOMER_ID

I have preset the form to the most common customer using the on load script.
CUSTOMER_ID.Select("INT");

However,when I load the page, the SITE dropdown is empty even though CUSTOMER "INT" has SITES. If I change the CUSTOMER, then the SITE dropdown gets populated correctly. If I change back to CUSTOMER "INT", then the SITE dropdown gets correctly populated.

Why does the SITE filtering not work when I first load the page? Can you please fix it?