With the update of 1.18 when I open a world, I get the one you want to update your world and I accept first I make a copy and when pressing update it puts creating world, it enters the world but it is not updated because there is still bedrock in layer 0 (already I have tried many times creating more backup copies giving it to update and the negative layers are not generated) 

They have told me that they may be connection problems but I have tried with all kinds of networks and checking in the comments of the playstore I realized that I am not the only person to whom it happens (they sent me here from an email so that give me the solution)