Hi,
This is a bit of a frustrating one. The mutation updateCustomer actually overwrites existing tags instead of adding to them.
The documentation should really be updated, or better yet the mutation should actually add tags instead of replacing them outright like it implies it does. Which is a lie.
updateCustomer Mutation Documentation
I understand a code change can take some time but at least consider updating the documentation. Partners could (and probably) are overwriting each others tags which could in turn affect app functionality that relies on tags.
But to answer my own question - tagsAdd will not interfere with other preexisting tags and actually adds instead of replaces.
Please Shopify hear my cry. Us devs need accurate words for operations at the very least! Save us from harming each other and ourselves!
I was pretty tired and frustrated when I made that first post.