When logging into my friends world online (my friend is on a Nintendo Switch also) I spawned in as if my player data had been reset. All my items were gone, and I all that was in my inventory was an unused map, as if I was spawning in for the first time. I looked around online for the last hour and couldn't find any reports of this issue.

It's worth noting that I loaded up an old server just before joining into my friends world, and just before that I had to re-log into my Microsoft Account. The only thing I can put it down to is my player data has somehow been reset. On PC I could probably access that data and fix it, but no such luck on a switch.

One final point, I checked my cloud save data on my Switch, and it gave me an error message reading that it had to restart it's upload due to an unspecified error in Minecraft.