Earlier quoted context omitted.
405 Method Not Allowed is trivial to fall back to POST. How do you know the GET request behaved incorrectly?
That's assuming the corporate proxy is well-behaved.
One should adhere to Best Practices since one cannot control every device between the app and the user. Best Practice says “GET has no body. QUERY can have a body. If QUERY fails (405), use POST with the body.” And eventually, enough proxies will behave well enough that at least the HTTP bit of the app has a chance of working.