That is NOT good. Although, knowing what my camera would see, I feel bad for anyone who would hack into my phones camera. They’d see the inside of my pocket, the ceiling of my car, the ceiling in the office, the bookshelf in my bedroom in the dark, the side of my face when I make calls… Nothing thrilling to see.
and this is one reason why I don’t have an iPhone. not to mention that they cost way to much for my budget.
I would worry about the bug. If I had a Facebook account, and used Facebook on my iPhone.
If you block mic and camera on an iPhone (privacy settings) for any particular app, than this is not possible. Apple’s API does not allow access to the camera or mic without explicit permission when you install the app. Personally, I never grant any of these permissions to any Facebook application. They are the last company on Earth that I would trust (including Google). I’m hesitant to even say Facebook and trust in the same sentence. And this case in particular is with Facebook Messenger, not the actual Facebook app.
I am a registered iOS developer, so I’m familiar with how these APIs work. iOS is a closed system.
If they’ve written about this today it means Facebook has been doing this for few weeks already…
Bugs, mistakes… they can call it different way. Technology is going to watch us always. 
I noticed this happening, I asked my wife if hers was doing the same, I have the iPhone 8 plus, she has the Xr, my daughters have the 8 and the Xs. Our iPads weren’t doing it so out of ours it was just mine. It hasn’t been doing it any more though.
Go to Privacy Settings for any Facebook app and disable mic and camera. Then it won’t even be possible. iOS is a closed operating system, so access to the hardware like the mic and camera has to be granted on a per-app basis after first launching a new app. Most people just click “Allow” to get through it.