Unity WebGL builds have a bug with POST request sending. It mangles request body in certain browsers on certain systems. For more info - see this Unity thread https://forum.unity.com/threads/mangled-post-requests-with-unitywebrequest-on-2018-4-x.858925/
This is a reference implementation of a wasm.framework.unityweb file patching. Place it in any Editor folder in your project.
It contains three methods that can be used to either manually patch the framework file after build, or can be included in your custom build pipeline as the last step.
PatchDevelopmentFrameworkCode - patches the unminified and uncompressed framework code. This is the "raw" code that UnityNative.js and UnityNative.asm.js files have.
PatchUncompressedReleaseFrameworkCode - patches the minified, but uncompressed framework code. This is what you get when you choose Compression Format - Disabled in the project settings of your WebGL target.
PatchGzipCompressedReleaseFrameworkCode - patches the minified and GZip compressed framework code. This is what you get when you choose Compression Format - GZip in the project settings of your WebGL target.
It uses System.IO.Compression.GZipStream API that doesn't compress as good as does the embedded 7za tool, but it's a crossplatform solution that doesn't require different code for process creation on Mac, Windows and Linux
Patching works by replacing the code that tries to send raw bytes over the http.send. It manually converts those bytes to a UTF-8 string using the TextDecoder functionality (Can I use it?).