Playing on a singleplayer world with multiple characters causes items,locations,xp,(all data) to be kept between both characters. Only on LAN will both characters maintain their separate locations and inventories.
How to recreate:
Start a singleplayer world with Character 1. Get some items, gather some xp, move around.
Close world.
Go on a second minecraft account (Character 2), and join the world you created with Character 1.
Character 2 will now have everything Character 1 had.
Open to LAN.
Both Character 1 and Character 2 with have the same items, xp, location, etc.
Thank you for your report!
We're actually already tracking this issue in MC-697, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki