Superflat world corruption when converting the world

Background story:

I've logged into my current superflat world to do some testings and I noticed that my world (that was generated in an older version of the game) would not generate the chunks beyond the one I've already generated before.

Worlds that are not affected:

• Worlds generated in 20w21a works perfectly (they generate chunks normally)
• Normal worlds generate chunks normally even after being converted

How to reproduce the bug:

1. Generate a simple superflat world in 20w20b (or older)
2. Switch to 20w21a
3. Convert your world to the current version of the game (20w21a)
4. The game won't generate past the chunks you've generated in the older version.

 

Screenshots attached (I've put the wooden planks myself to see if I was able to place blocks in those chunks)