Sure enough all the API requests for data were coming through, but whenever a request for image happened - nothing would hit the servers.
What the heck I thought to myself?
I said to the client 'that can't be, that's almost impossible....the only way that's possible is if the SSL traffic is decrypted, inspected, and images blocked from being requested, which, is a MITM attack".
He redirected me to his IT provider. I phoned them up, and explained the situation.
"Ahh so they're _____"
Me: "So what does that have to do with the price of fish?"
Them : "Content filtering..., you need to talk to ____"
Sure as the day is long, the content filter was a VPN all members of ____ had to have on their mobile devices (I don't know how widespread this is, whether it was just this business, or the entire ____ )
I applied to have our system approved, it was, and just like magic the next day photos started coming through.
I'm guessing basically it detected any .jpg/.mp4 etc URL's in https requests and flagged it up and blocked them from being requested. You can be sure on those devices the VPN would have been somehow locked in with device management, and there's no way on gods green earth they were getting at Facebook/insta etc.
So, it's not just meta. That really hammered home how seamless it can be to end users that they really can't trust what's actually happening on their devices.