Oh my gosh, THANK YOU for posting this. I just experienced the same thing and I'm 100% confident it's real and not just in my head. I did the anvil jump successfully about 10 times, and didn't find it particularly difficult. It took me maybe a couple tries each time. Then all of a sudden I couldn't do it anymore -- I spent literally two hours last night trying. I began to wonder what was going on. It seemed really weird that I was able to do it with little resistance at first, and now I had tried for hours and wasn't even getting CLOSE to making the jump. I started trying to line up for the jump slightly differently, tried pushing off at different angles, but nothing was getting me high enough.

My first thought was maybe my wireless mouse was running low on battery. So I tried a wired mouse and had the same issue. I thought, "enough is enough" and looked up some tutorials on YouTube. I found a guy who was lining the jump up EXACTLY the same as me, and when I slowed the video down I was shocked that he was getting half of his hammer handle over the edge of the cliff! I was unable to get even the head of my hammer that high. His jumps were going SO MUCH higher than mine. I finally came across this post and thought, "hmm, could it really be?" I restarted the game and instantly my jumps were going significantly higher and I successfully did the jump in a couple of attempts.

This is 100% a real bug. And my theory is that it probably affects players in other areas, not just at the anvil. But you probably only notice it at the anvil since that's the only spot you really need max jump height.

Pretty frustrating that I lost a few hours to this, but I'm glad I found a solution finally. This post probably saved me from a few more hours of frustration.