My sons and I were playing a world. Jack created the world on "his" computer, Michael and I joined from ours (all three of us have accounts). We've played a few days in the world, mostly all three of us at the same time, once or twice just the two of them. Yesterday I replaced the computer that Jack was using. Installed Minecraft and copied the world over to that new computer. Since that computer is the "all purpose" computer in our house, I logged into all three accounts in that launcher (since it supports it).

Today, I decided to make sure our world was working. On that new computer, the one that holds the world and Jack usually plays from, I:

1. In the launcher, switched to my login
2. Hit the Play button (1.16.2 was selected - the only version on this new computer)
3. When Minecraft launched, selected Single Player
4. Launched the world

When I got in the world, I noticed things were odd. First, I didn't have the equipment I remembered having. I seemed to have a combination of things I remembered having in my inventory plus the things I remember Jack having (he was the only on carrying a clock). Not realizing what had happened, I put a few things away in some chests. Then I hit F5 and noticed I was not in my normal skin (I was in the default). And lastly, I had only 4 XP. That was definitely where Jack was. I was the one doing all the "scary" mining, so I had like 30 XP. So, I:

1. Closed the world and exited Minecraft
2. Opened the launcher
3. Noted that my profile was selected
4. Selected away from my profile; selected back to my profile (not that I expected that to do anything)
5. Selected my skin
6. Launched again
7. Loaded up the world as before

I still had the same XP, the same inventory (what I still had after putting some stuff in chests), but I was now in my skin. So, I:

1. Closed the world and exited Minecraft
2. Switched to Jack's profile
3. Launched
4. Loaded the world and opened it to LAN (as we usually play)

This time, it was clear it was Jack. He had his skin (without me having to change that in the launcher after having chosen his profile) and the XP and inventory from the past two launches.

I went to my computer and connected to the game. Clearly at some point our data had merged, at least partly. I had 4 XP. I also had exactly the inventory Jack had - a complete duplicate.