The A7S2 has shot the footage at 120 fps, when the frame rate is altered to 24 for playback to achieve the slow motion effect, Cortex seems to be unable to play the footage (timeline moving , picture still frame.). If rendered out the footage has a 5 to 1 frame cadence, which makes the clip look stuttery and it is 5x longer than it should be.
Did you change the play speed or edit the frame rate in the “Edit Timecode” window?
Play speed, apparently edit timecode works as I would expect.