I made a comment on MC-142134, "Actually confirmed in 1.14 pre1, I downloaded my multiplayer world back to my computer, put it to singleplayer and run the "Optimize World" function in 1.14 pre1. Uploaded it back to my host, and joined the game. A lot of the originally mislighted areas were fixed but this is new: (refer to lighting again.png)"

I did not realize other problems apart from lighting bugs after optimizing the world, but apparently it reset some of the chunks. I don't think this is supposed to happen.
As you can see in the picture, a part of the city built was gone.

Edit: I believe this happens with just loading this older world (created in 19w18a, chunks correctly showing in 19w14b) in 1.14 pre1. Meaning that purely by loading the world in 1.14pre1 some chunks were regenerated and overwritten. I tested this with an even older backup used in 19w14a, and by loading it in 1.14 pre1 singleplayer, those chunks were regenerated as well.

(The world is almost 1GB)