I logged into my partner's Minecraft Realm on Nintendo Switch, and was just where I left my character, with all my items. I let the Switch go into sleep mode, and later logged back into the Realm, but this time on Windows 10. I was dropped at the spawn with none of my items. As well, all of my items were gone from my inventory, and my Ender Chests.

I did notice on the Switch the "Taking Inventory" achievement popped, despite the fact that I definitely already earned it on the Realm.

Let me know if I can provide any further details. I'd like to help this not happen to anyone else.