Every time I opened one of my save created in 1.12.2 and edited during the 1.13 snapshots myself goes back to a unchanged place with the same facing direction, and the inventory nbt of the player is also going back to the origin condition, which is lastest seen in 1.12 versions. I will explain the reason why I know that it comes to the players data from 1.12. the 27th screenshot is the first time I opened the game after I found the bug, the 28th screenshot is showing that I have put a command block there, the 29th screenshot is I opened it again, the 30th screenshot is showing that the block is saved, the 33rd screenshot is that I have cleared my inventory, the 34th screenshot is what my inventory becomes after I open it again. the 31st screenshot is showing the name of the bow, which actually has a nbt that has the format of display:

, well that is because this version is edited in 18w16a, and the display nbt's format is changed, so I have changed the nbt, but in 1.12.2, it shows something like this(I have opened it in 1.12.2 accidently), and I has edited it for weeks after that, so there is nothing in my inventory in 18w16b
1.created a new world in 1.12
2.get a diamond block
3.go somewhere else than the spawnpoint
4.place it under the feet
5.quit and save(do not clear the inventory)
6.open it in 18w16a
7.place a dimond block somewhere else a
8.quit and save the game(or the bug might not occur, I don't know)
9.open it in 18w19a
10.go somewhere else than on the command block
11.clear the inventory
12.quit and save the game
13.see if the player is on the diamond block and if there is a diamond block in the inventory
14.try something else that is related to the playerdata file