Of course, Facebook says its a bug. These bugs always lean in one direction-towards more invasive spying, rather than less:

When you’re scrolling through Facebook’s app, the social network could be watching you back, concerned users have found. Multiple people have found and reported that their iPhone cameras were turned on in the background while they were looking at their feed.

Source: Facebook bug shows camera activated in background during app use – CNET

I deleted all Facebook apps in 2018 after the depths of their global surveillance platform were well publicized.