why does protobuf uint64, fixed64 map to string when converted to JSON

It's due to the way that all numbers in JSON are basically IEEE-754 64-bit doubles... which means that any 64-bit integers can lose information. Using a string instead, all 64-bit integers can be round-tripped with no loss of information. 

https://groups.google.com/g/protobuf/c/4-BY-k-Lk-g