After converting my 1.12.2 world into 1.13 (and using 'optimize world' option afterwards, twice for good measure) I've noticed that some of the items got switched - in double chests contents went from up to down and vice versa. All items remained untouched, so that's not that much of an issue. However, after that I noticed one of my chests disappeared; upon closer inspection, it turned out to be invisible (it had a hitbox when I hovered my mouse over it). After opening the chest it's appearence turned back to normal, but inside - it was empty. Good thing I made a backup, this way I was able to identify what was in that chest before conversion - it was full of saddles (maybe the fact that it's one type of item is important, i don't know).

Edit: Here, I'll write it in some less novel-like way:

What I expected to happen was...:

I expected the chests to be in the same places and contain the same items in the same slots after converting my world from 1.12.2 to 1.13.

What actually happened was...:

Some double chests had their inside flipped upside down, one chest become invisible; after opening it, it turned back to normal, but it was empty - in 1.12.2 it was full with saddles.

Steps to Reproduce:

1. Create a world in 1.12.2.

2. Place down a double chest.

3. Fill it up with items.

4. Convert the world to 1.13.

5. Use 'optimize world' option afterwards (I did it twice).