By reinstalling Minecraft after exiting the beta, I lose all my worlds. To replicate:

Step 1: Make backups of your worlds

Step 2: Join the Beta (look it up)

Step 3: Delete and Reinstall Minecraft to get the beta

Step 4: Make a new world and play around in it (not sure if it changes anything, but I did it so I just want to put that in)

Step 5: Leave the beta

Step 6: Delete and Reinstall Minecraft to exit the beta

Step 7: Notice that your worlds disappear