Also, we don’t know when exactly when Luke’s old saber was found. Probably after he left. Lightsabers, (and the skills to make them) are a rare thing in the galaxy, so whoever found it on Bespin probably tried to sell it.
Let’s think this through, here. Luke drops his lightsaber on a gas giant planet, one very much like the planet Jupiter or Saturn. So it would fall and fall and fall until it hit the planet’s solid surface…
…which is as hot as the surface of the sun!
So how, exactly, did the lightsaber survive? Never mind, for now, how Maz got it.
I know, I know…it’s fantasy. Anything is allowed now in Star Wars. Anything at all. To explain anything, just say, “The Force,” “It’s a movie,” or, “It’s different than our universe.”