User is trying to enter the Social Security Number (SSN) of the W-9/1099 vendors in the Tax ID field on the Vendors > General > Dates & Hours screen, but Core does not save it in the proper format (AAA-AA-AAAA).
You can customize the format and mask of such fields in Core, say the Tax ID field, in Global Settings > Custom Labels. In this case, go to its Vendor drop-down, set the Format field for Tax ID as 999-99-9999, and click Save. The information now displays in the proper format. Check Core Help Center for details.