Inventory wiped on server

First time inventory wiped on server: I changed my skin, left server and rejoined to reset the skin, inventory wiped, spawned at spawn location.

Latest inventory wipe: I broke some blocks in the nether and placed some. When I left the nether and returned to overworld, it was like I joined the server as a new player, spawned in spawn location with empty inventory. When I went back into the nether, all the changes I made were gone, it was the similar to before. Maybe a server sync issue?

Either way, it seems like there is some issue with the server identifying users. 

I'd like to add, I had been into the nether after changing my skin and returned to the overworld, my inventory was not wiped, I cannot confirm if my changes to the nether were kept but I believe they were.