Old Worlds totally corrupted

I’ve a load of old worlds that are instantly corrupted upon opening.

It seems anything created over a few months ago will not open. This is most distressing!

Edit: Reverting back to 1.14.3 loading the world & saving it then reinstalling Beta and finally opening the world fixes the issue - but this is not a solution

 - However this DOES NOT stop the other dimensions from becoming corrupted!! (see video)