Starting with the Caves and Cliffs Part 2 update, Minecraft will make required Android storage updates that may make player worlds disappear. The worlds are still on the device, but they must be moved manually for the game to continue to access them.
Worlds may disappear in these conditions:
The world is saved on External Storage (Settings > Profile > File Storage Location > External)
The game is uninstalled and reinstalled or the user backs up and restores to a new device
If you find yourself without access to your worlds, there are two approaches you can take to recover them. Due to the differences between Android vendors and implementations, you might need to try both approaches to find one that works for you.