Apple Quicktime cannot open a video that has been brightened using ffmpeg?
Pixel data in videos is typically stored as YUV, whereas colorlevels works upon RGB format pixels, so it tells ffmpeg to convert it beforehand. After the filtering, the reverse conversion to YUV before re-encoding uses a YUV format not compatible with basic players, so one has to force conversion to a compatible format.
ffmpeg -i ORIGINAL_VIDEO1.MP4 -vf colorlevels=romin=0.4:gomin=0.4:bomin=0.4 -pix_fmt yuv420p -c:a copy BRIGHTER_VIDEO_1.MP4