Whenever you make a call to HighLevel to "upsert" a contact, the current behavior is that any tags that you pass will overwrite/replace (ie. remove current tags) that contact's tags instead of adding the tags that you pass.

Upserts should be additive and should not remove, overwrite, or replace elements from objects, but rather insert them as necessary.

Remove current tags - BAD

Add more tags - GOOD

Another way to do this would be to add a junction (aka "pivot") table for tags that belong to a contact and just add the tags to a junction table, but since we're placing the Array object "Tags" directly on the contact's record instead of creating a separate object and referencing it (like you would in an RDBMS), the "Upsert" function should do something like the following: