Nether and End is reseted while changing versions from 1.16.4 to newest snapshot

I changed version from 1.16.4 to 20w46a, and my whole nether realm got reset. Backing up save state with whole nether still won't help to fix that. To fix that i need to change version back to 1.16.4 and then backup the save, and can't change back to newest snapshot because it will reset it again.