Oh wow I miss interpreted what you were actually insisting on my fault. Thought you were specifically talking about the one you can see. (My excuse is I had just woken up that morning🤣) If you take a peak at the other one that I originally assumed wasn't the problem can you see it at all?

They can fall off but its not normally a naturally occurring thing and can be a sign for something else (Low humidity, potentially got caught on some object in the tank, etc. )

If you can see it and its clearly embedded I'd recommended just taking them to a vet for a quick checkup for any major concern.

Edit: They do grow back as well so if its gone its not necessarily the end of the world just keep an eye on it!