Relogging in from standing on half slabs underground

Underground base made from cave, using half slabs as floors to deter mob spawning. When logging back in, will respawn on the surface, a few times it has knocked me into the next room, twice it has spawned me in a wall.(suffocation). This has happened daily since version release. Stone, cobble, stone brick and nether quartz slabs is what I have been using. Seems to occur with them all.