Empty request body with content-length != 0

What could be the reason for SOMETIMES getting an empty request body?

I'm using request()->all(), even when the array is empty the content-length is something different than 0...

//request()->all()
[2017-07-13 14:20:07] production.DEBUG: array (
)
//headers
[2017-07-13 14:20:07] production.DEBUG: array (
  'content-type' => 'application/json',
  'accept-encoding' => 'gzip,deflate',
  'user-agent' => 'Java1.7.0_67',
  'host' => 'XXX.com',
  'accept' => 'text/html, image/gif, image/jpeg, */*; q=.2',
  'connection' => 'Keep-Alive',
  'content-length' => '225',
  'x-forwarded-proto' => 'https',
)

This doesn't happen always which seems odd... Maybe 1 out of 5000 transactions. This is a webhook used only by one provider and there are some patterns that repeat themselves when this happens (i.e. out of 30 cases, these all belong to just 10 users, some with 1, 2, 3 or 4 cases) so I'm almost certain that the error is on their side (provider).

Is there something I can do to prove this?

Edit: I've been thinking about enabling the apache dumpio module, will this do any good? Or will I bee seeing the same empty body with content-length > 0?

http://httpd.apache.org/docs/2.2/mod/mod_dumpio.html