Hey @GOAT_GROUP - I was able to look into this for you and have an update! I spoke with members of our product team responsible for the metafields component and they confirmed that at the moment the mutation doesn't allow the use of the validations field, but that they will work on adding that functionality in an upcoming release for the API. I can't guarantee a timeline, but I can confirm that this is part of our near-term roadmap.
Thanks for mentioning this and bringing this to our attention - it's really appreciated. Let me know if we can clarify anything on our end.