I play on Nintendo Switch (because the Xbox One version is broken and takes 15 minutes to load, fix your own console Microsoft! See MCPE-79644) and I have been having issues joining multiplayer worlds. With or without the "Invalid Game Session" tag (Another huge issue for Switch players, see MCPE-155025), when I join my friend's (PC) survival world I load in at the world spawn location with my inventory completely wiped. Sometimes I am able to fix this by exiting the world and rejoining, which returns me to the location at which I exited the world previously and my inventory returns to me. However, other times I need to restart the game, my console, and rejoin the world upwards of 20 times before my character loads correctly. I am connected to the internet, my Microsoft account is linked properly, and all factors are working as intended that could affect my character not saving the data of world position and inventory.