If the source was a mono film, they should be, but there are quite a few films that were mono in theaters but remixed to stereo for home video: https://www.hometheaterforum.com/community/threads/the-ultimate-list-of-mono-to-stereo-remixed-soundtracks.339481/
Quite a couple of the films on that list had certain elements (namely the music stems) recorded in stereo, but downmixed to mono for the optical soundtrack on a film strip (which was, obviously, monaural). Just saying…