I am using the new launcher.

When closing a Minecraft (singleplayer) world, it seems that the player data is not saved, as when I rejoin the world, I am placed back where I stood when I last saved the game in the old launcher, the inventory is also set back to the previous one. All blocked placed/destroyed work fine and stay in the world. Worlds created in the new launcher just place the player back at spawn with a clean inventory (as if they were joining for the first time).

This issue seems to have gone away, I haven't played in a while, tried joining my world and now it just works. I don't know if this post had anything to do with it but thanks for looking in.