Upon entering a world in 18w19a I entered the last time in 18w16a, my player inventory (incl. worn armour or a block on my head like e.g. a carved pumpkin), gamemode, effects as well as my character's position and full rotation in the worldsave gets always reset to one specific point. This solely applies to the player, blocks which were already changed in that world, or e.g. dropped items (which are below the despawn time) persist if you re-enter that world.

If the player had effects applied (in my case it is nightvision) this effect is also persistent, even if you removed it before you logout and reenter the world.

It only seems to happen in:

• worldsaves which I entered the last time in 1.12.2.
• worldsaves which I entered the last time in 18w16a, but it does, for example, not happen in a world I entered the last time in 18w05 as well as not in another world of 18w01.

(haven't checked 1.11 and 1.10 yet)

Video showing the issue:

Also, I'd like to add that any builds you've made in a world which you entered the last time in 18w16a vanish upon entering that world in 18w19a (as well as 19b, but this is known to Mojang [ProfMobius tweets, just mentioning it for those reading this bugpost and not knowing]); but if you enter in worlds you entered last time in an older 1.13 snapshot version, e.g. 18w01, the builds will still be in there.