If Face ID isn't working on your iPhone or iPad Pro Q&A
A few drops of water fell on my iPhone 11 Pro Max when I used it next to the lake while still wet from swimming. Strangely, my waterproof phone in a protective case with a screen cover, albeit which doesn't cover the notch gave the warning:
A problem was detected with the TrueDepth camera. Face ID has been disabled.
The help documents at Apple go in circles, each one leading back to the other. Those documents talk about updating the OS (which strangely enough by faithful iPhone 11 had been working under similar conditions with no issues on iOS 13.7.2). Restarting the phone did not help. Adding a new face in Face ID did not help. The Apple documents recommend visiting a repair centre which didn't sound like much fun or really necessary.
What did work was removing Face ID and then restarting the phone in Airplane mode and re-adding Face ID.
I wanted to leave this answer on this thread, but it was closed to new responses and suggested starting a new thread.
I hope this info helps someone.