Slash commands configured to receive a GET request receive the payload in the body of the request instead of being encoded in the query string. No Content-Type field is specified.
While this is not strictly forbidden in HTTP, it is highly unusual and likely not supported in some stacks. The body of a GET request isn't supposed to influence the server's response.
It's possible this is due to Slack compatibility. Investigation required.
Likely will require a developer to setup and test using a plugin.