Stereo sound like we know from music was never used in cinema. There are lots of article if you search in books or google, I can explain you the main facts: In the past, the first audio format was mono with one speaker in the middle of the screen. This format is still the most important channel if you imagine the situation of a cinema. Wherever you sit in a cinema, you will hear the things from screen 'out' of the screen. So 'stereo' as we know from music is not recommendable in cinemas: it's always focused on hard left or hard right. Imagine if you sit left, you only hear the things from left very present and it's not coming out of the screen. This is weird and irritating, so this format makes no sense in the cinema. Later, if the format extended into multichanels, the centerchanel still is the most important channel from all.
The next format was Dolby Stereo with 4 channels: Center, Left, Right And Surround and was used a long time until the digital format brought more and more channels. But the situation always stay's the same: the sound should come out where the action is, and this is from the screen and should be as realistic as possible. I tell this as a 'surround guy' (see https://www.soundeffects.ch/) Surround is great but should never take the action from the screen, except you really want it. Otherwise it's bad.
So if you make a soundtrack for the big screen, it's better to mix it Mono or do it in Dolby Stereo than 'Music-Stereo'.