Inventory Dissappeared without warning
Solution 1:
It's very possible that whenever you leave the server and rejoin, then the game thinks that you restarted it. It's never happened to me on Minecraft specifically, but for me it is a very common occurence in games. A few years back I was playing a Batman game but every time I turned off my playstation or quit the game, all my data got erased. No corruption, the game just thought I was restarting.