For HTTP purists, GET is not supposed to have a body. Fielding has noted from his perspective there is no valid scenario in which GET should have a body. “So, yes, you can send a body with GET, and no, it is never useful to do so.”
There is some interesting discussion about this at rest - HTTP GET with request body - Stack Overflow
An item there notes: “…sending a payload body on a GET request might cause some existing implementations to reject the request.”
I recently had been working with a GET that had a body. Using API GW and an internal IS as the host of the “native” service. I could not determine which of them was dropping the body. I switched to have it use POST so did not dig further. But it would be interesting to know if GW and the rest directive path of IS can be coerced to pass the body (as a stream) to the _get/_default service, if indeed one of them is ignoring it.