I was messing around in the nether, and when I went to walk around the soul sand valleys, I jumped up onto some soul sand but was stopped. As i continued to try different things, I have come to the conclusion that there is a bug with jumping up onto soul sand where you get stopped. You can jump again to get past it, but if you stand on that block, not being completely surrounded, you will be pushed off. I also noticed that this happens with path blocks and farmland as well, and it most likely has to do with these no being full blocks. It almost seems like there is a invisible entity of some kind pushing you off.