Empty, invisible chests when converting from 1.12.2 to 1.13

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).