Strange mask behavior when using Alt+J and Alt+U

Hello 🙂

When I'm in the Compositor window (to edit the different points forming a mask) and I've recorded several keyframes frame by frame to advance the mask (I press F3 to bring up the mask), if I use the keyboard shortcuts Alt+J to advance one frame or Alt+U to rewind one frame, the mask doesn't really follow the frames! In fact, I have to advance two frames (2x Alt+J) and rewind one frame (Alt+U) to get the mask in the right place on the right frame. So if I only use Alt+J to position the mask points on the next frame, there's a strange behavior... I hope I've explained myself clearly; could you shed some light on this behavior?

(However, when I move the images forward or backward by clicking on the keyframes in the track, everything works fine.)