RFC 7951 is quite explicit [0] on the requirement for int64, uint64 and decimal64 types to get serialized as JSON string values (so inside quotes, unlike other numbers).
Restconf outputs numeric (as opposed to string) values for int64, uint64 and decimal64 , so it is not RFC compliant in this sense.
[0] https://tools.ietf.org/html/rfc7951#section-6.1