I simply joined into a friend's world (I believe they are on an Xbox S) and when I joined, my inventory was empty (No armor on, or shield in offhand, it's fully empty like when you join a world first). My inventory was filled to the brim when I last was in the world. Here are some extra little tidbits which may or may not help:

• When I rejoined, I was in a Nether portal.
• My skin reverted back to a default Alex skin, whereas previously it was the fancy enderman skin with the pancake cape.
• I have recently been having certain issues, like my friend having to invite me rather than me seeing his world in my friends tab, and my Minecraft loading improperly in various manners (Invalid session, also when I don't close Minecraft but put my switch to sleep, when I turn it on again it says a communication error has occurred and my Minecraft has to reboot to make it function properly)
• I had just come from the nether to the over world.
• We logged off the world quite promptly after I left the nether.

P.S I had another issue occur before this, where when I joined I had no items and I was back at the world spawn. I already had a very empty inventory however, so I can't confirm with certainty that items were deleted that time. I did have my respawn point set to a bed far away, and I had played on the server many times before with no issues. I think these might be interconnected.