You are adding your keyframes in the viewer, right?
When you click on the Transform icon you will notice that the skimmer disappears and the playhead turns pinkish, like the scrubber and playhead have become one.
I don’t really consider it a bug, its due to the fact that you can skim samples.
Zoom in and click around and you will see that the frame area is sometimes in front or behind the playhead. You can also move the playhead inside the frame area, since you can skim at sample level, but video keyframes are at frame level.
At times the playhead is right in the middle of the frame area. So if the frame area is on top or before the playhead, the playhead jumps back and the keyframe is set one frame before.
“Always look on the bright side of life” – Monty Python