I just wanted to confirm if you have you tried the example formatting used in the documentation with the most recent API version, 2021-07? Your request differs slightly, so I pulled up the examples below of the format I was able to successfully complete the mutation with.
Additionally, you may want to query the ShopFeatures object and confirm if “internationPriceOverrides” is set to “true”.
If not, because some price adjustment features are associated with a Shop Plan (per our documentation here), I would suggest having a merchant contact support for more info. If yes and you are able to replicate the issue using the request format above, please feel free to share an example X-Request-Id header returned from a request with this functionality.
With that I would be happy to take a look and pass back any insights I can - Cheers!