Confirmed.

After every unity build to Xcode Project,

@synchronized (self) needs to be wrap the contents of writeMoreBody function.

From within the Xcode project file Classes/Unity/UnityWebRequest.mm

Just an update. A bug report has been submitted to Unity regarding the UnityWebRequest usage on iOS devices on the Unity 2019 engine.

Bug Information : 1163795 | (Open) iOS - UnityWebRequest Errors - Large Packets - With Examples

Zipped Project : https://www.dropbox.com/s/ccybrg3tirpud0u/iosWebRequestFailures.zip

To Repro:

• Networking Error observed on iOS devices. Run Example from Editor, Tests will commence immediately. UnityWebRequests of varying sizes ~100b, ~1200b, ~8000b, ~21000b and ~48000b are sent multiple times during the tests. The messages are sent to an echo endpoint, in order to confirm proposed send, and processed server data.
• Logs are outputed to the screen, as well as the console for larger error packets.
• All tests pass on Editor.
• Build for iOS.
• Open Xcode, Autosign the app, and build for device.
• Tests fail at inconsistent moments in running tests.
• Close the app, Re-run tests. Observe different failure conditions. Repeat.

Observed errors. No error, or data response returned. (0b response)
Observed errors. Corrupt data sent. (misaligned data)
Observed errors. Corrupt data sent, varying response from server (different sized response, shorter and larger response size)
Observed errors. CRITICAL networking error, where NO MORE network traffic is processed. (need to hard close the app for Network traffic to occur again)