Custom Field automatically updating when re-saving record

[Zach Oliver]

We have a custom dropdown menu field on the Product - Site (ITMFACILIT) level with a handful of options on it. We have an issue where if we create a new product-site record for a product, that field will remain blank, but if we update seemingly any other fields on that product-site record, our custom dropdown field will automatically update to the first available option on that menu (even if there is a default value entered on the ITF screen for that field). 

For example: We set a default value of the 2nd option on the dropdown menu for that field on the appropriate ITF screen. We created a new product-site record for a product. When opening that new product-site record for the first time - that custom field is blank, so none of the options on the dropdown menu are populated there. We then clicked into another random field and typed something just to trigger the Re-Save option for that record. Re-Saving the record (whether or not we actually add new information to another field) causes our custom field to then populate the 1st option on the dropdown menu, and not the 2nd option like we set as the default value. 

What could be causing this to happen? Has anyone else had similar issues with custom fields before?

Preferably we would want that field to update to the default value that is set at the ITF screen level when the product-site record is created. Or, at the very least, remain blank until manually updated and saved by a user.