I have tested this (very) simple server action in Odoo 12 that would use Python to add a tag to a contact. In 12, it works whether or not the contact already has a tag. In 13, it will overwrite whatever is currently in the tag field.

How can I modify this so that it appends rather than overwrites?

(99 is the ID of the tag in my test.)

record['category_id'] = [(99)]