So it turns out the at API endpoint exposed by Microsoft will only except dbug symbol files upto 256meg using the appcenter plugin.  Ours iOS  app.dSYM file recently grew to 257meg and was rejected with an error message of  

io.jenkins.plugins.appcenter.AppCenterException: Upload symbols to resource unsuccessful: : HTTP 413 The request body is too large and exceeds the maximum permissible limit.: <?xml version="1.0" encoding="utf-8"?><Error><Code>RequestBodyTooLarge</Code><Message>The request body is too large and exceeds the maximum permissible limit.
RequestId:92c2b0f6-601e-0057-308e-edc32e000000
Time:2020-02-27T16:56:23.4809866Z</Message><MaxLimit>268435456</MaxLimit></Error>

When I inquired with MS about this they said its a limitation of the api and you would need to use the chunk upload approach to upload files larger than 256meg.  

I don;t know if this was known before as I can find no mention of upload size limits anywhere in their documentation and it also does not seem to affect the .app uploads just the debug symbols upload.

The current work around is to stop uploading with plugin and after each build hand upload the .dSYM file through the website. (Which strangely enough does not exhibit the same problem)

If you need any further info please let me know.

Thanks

Terry