I made the sound, music for a short film (a stereo 48k mix).
The film had no dialogue. I noticed though in the screenings (2 film fetsivals) that the stereo image was almost gone.
So i am wondering if this is a problem with the format it was delivered (thus stereo) or if there was some encoding-decoding to something different from the festival that messed with the soundtrack.
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'.
Most festival/venues i've been to, tend to mess up (stereo) soundtracks. Not sure if this is the case ofcourse. Did you hear phasing issues or was it just monophonic? Where did you mix your short? Most of the time is has to do with the setup (i've seen very old speakers and cheap mixers in the projection room) or the dolby matrix 'compatibility' of your soundtrack.
Did you do a soundcheck? I've been doing this everytime and saved the soundtrack multiple times from an unholy monophonic meltdown.
Also try and search a bit more in SSD, this has been discussed before, so there's a lot of information already here.
In my experience in presentation and film festivals, essentially we prologic any stereo content. Stereo in a cinema is bad because the stereo phantom centre will move depending on where you're sitting, so the tradeoff is made so centre information like dialogue is always actually coming from the centre.