Beta Tester - Lost world after update

Hallo
my son is minecraft beta tester.
Name: Fullmontain937
 

 
 
After installing an update, all created worlds disappeared.

I checked the internal folder where the worlds are saved, but as you can see in the attached image all the 15-20 worlds he had created have disappeared and you can only see the 2 new worlds created when the program is restarted.

Why did they disappear with the update? Do you happen to have the ability to recover online worlds?
What internal folder could the old worlds be saved in?
thanks for the help
Ale