Sydney trains realtime headers missing

Real-time Public Transport
1

for all realtime endpoints you can get the last-modified header
like this:

{
  "last-modified": "Thu, 11 Jul 2019 04:15:31 GMT",
  "content-type": "application/x-google-protobuf;charset=UTF-8",
  "cache-control": "private",
  "expires": "Thu, 11 Jul 2019 04:15:46 GMT"
}

but for /sydneytrains

{
“cache-control”: “private”,
“expires”: “Thu, 01 Jan 1970 10:00:00 AEST”,
“content-type”: null
}

the content-type, last-modified, and expires headers are missing,
this is for trip-updates/vehicle-positions/service alerts