I have been puzzled about it too, but now I think I have an idea.. maybe.
When you zoom into a step to set the event, technically speaking you are looking at a step "position" filled with some step data, which corresponds to the step of course.
When the position event is active and the sequencer is running, the step data gets updated to the data of the step that moves into the position you are looking at. So that may be the cause of the trouble. In earlier OS versions the view would not follow the data but stick to the physical position.
That makes it seem like the step data is changing by itself, but what you are actually seeing is the step data set being swapped altogether.
What OS version are you on? With that info I can track for sure if this is the cause of the problem or not.
Cheers,
Gabriel