I really hate when people try to use individual player's +/- in a vacuum to draw conclusions about that player, when there's 9 other variables going into that stat every single game.
We have weak defenders all over the perimeter that Gobert keeps from getting exposed because he's the generational talent on defense. Why do you think we routinely get torched in games when teams can successfully get Gobert out of the paint? Why do you think Tony Bradley suddenly wasn't such a bad defender during his time in Philly? It's not Gobert getting exposed, it's our perimeter defenders.
Similarly, when Gobert is sitting, it's not his backup getting exposed, it's our perimeter defenders. Favors is the only player on the team that doesn't share any of his minutes with Gobert. Our perimeter defenders get exposed without Gobert to protect them, and +/- watchers who don't understand what's happening just assume it must be Favors' fault.