I don't think that we can just blame this on the GHL API, because this workaround works ...
If the Tags section of the "LeadConnector V2 : Create or Update a Contact" action is left Blank, and then a "LeadConnector V2 : Add Tags to Contact" action is used to add Tags. Then, everything works.
The current Tags are maintained, and the new tags are added.
So, Pabbly can do it!
The question is, why doesn't the Tags section of Pabbly's "LeadConnector V2 : Create or Update a Contact" action work correctly? It just needs to work the same way at the "LeadConnector V2 : Add Tags to Contact" action.