If a product has a non-hyphenated product tag, e.g. “Testing Tag” (and saved).
Then you try to add a hyphenated version of that tag, e.g. “Testing-Tag”.
The hyphenated tag is not added.
This is both in the Shopify admin and through GraphQL. No error messages appear for either.
Shopify seems to “handleize” the key for the tag and use that as the identifier.
So after “Testing Tag” is created, the key for that tag is “testing tag”.
If you then attempt to add the tag “Testing-Tag”, you guessed it, it’s treated as “testing tag” and I believe will just add that tag instead.