JSONSerialization re-encodes Decimal as Double

[blindspotbounty]

I am using JSONSerialization to sort keys for arbitrary JSON represented as String:

func sortedJSON(_ string: String) -> String? {
    guard let data = string.data(using: .utf8),
          let json = try? JSONSerialization.jsonObject(with: data, options: []),
          let sortedData = try? JSONSerialization.data(withJSONObject: json, options: [.sortedKeys])
    else {
        return nil
    }

    return .init(data: sortedData, encoding: .utf8)
}

/// example of usage
enum DecimalContainting: Codable {
    case decimal(Decimal)
}

let decimal = DecimalContainting.decimal(Decimal(string: "129.9")!)
let jsonEncoder = JSONEncoder()
let data = try jsonEncoder.encode(decimal)

let jsonUnsortedString = String(data: data, encoding: .utf8)

let jsonSortedString = jsonUnsortedString.flatMap { sortedJSON($0) }
print(jsonUnsortedString)
print(jsonSortedString)

The output is surprisingly different for sorted and unsorted data:

Optional("{\"decimal\":{\"_0\":129.9}}")
Optional("{\"decimal\":{\"_0\":129.90000000000001}}")

Notably, this happens on MacOS but if I run the same code on linux, the output is the same for both:

Optional("{\"decimal\":{\"_0\":129.9}}")
Optional("{\"decimal\":{\"_0\":129.9}}")

Swift version macOS:

$ swift --version
swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)

Swift version Linux:

$ swift --version
Swift version 6.0.1 (swift-6.0.1-RELEASE)