Step 1 just power off your phone.
Step 2 open i again
Step 3 uninstall fb
Step 4 install fb again
Step 5 if it doesn’t work then buy a original one and if it doesn’t work again just destroy all iphone 11 and that is all
The same thing happened with me couple of weeks ago! I think it’s the processor what gets hot. If this thing occurs, close down all your running tabs that’s open behind and leave it for a while without using.