I've revisited this and I think I see what you are referring to. It is somewhat subtle what is happening, but let me explain my findings and see if you are experiencing the same.
While moving a point on the tone curve around with my Wacom stylus, sometimes the point will refuse to move or move somewhat erratically. What I discovered is that my cursor and the point are not moving together. This is especially true when moving upward or to the right. Downward and to the left have a lesser effect. To illustrate this, click on a point that is low on the curve and move it upward. When you let go, the cursor will reappear and it will not be on top of the point. Instead, the cursor will have moved further up. Now click this same point and move it back down. When you let go this time, the cursor will be much closer to the point, if not directly on top of it. The same happens when moving to the right when compared to moving to the left. The movement to the right is really significant because the cursor will hit the edge of the screen very quickly, preventing you from moving the point further to the right. The same can happen when moving the point upward, especially if you repeatedly move the point up and down. This limiting of the movement to the right or up can make it seem like the point isn't doing what you want and it only gets worse the harder you try. This can also make it likely that you'll end up moving the point in some direction that causes the point to become invalid and get removed (moving it off the graph or past another point on the graph).
If you can verify this is the behavior you are seeing as well, then we can get this reported to engineering for repair.
The workaround is to only move points a little at a time before letting go of and re-clicking the point, especially when moving up and to the right. Big, sweeping movements of the stylus while clicked on a point should be avoided.
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