The film was scored for a big band plus a string section.
Elmer apparently didn't know that when you wrote straight eighth notes for jazz guys, that they would automatically swing them.
So, anything with a swing feel that should have been written in 4/4, Elmer wrote in 12/8, with a tie between the first two eighth notes of the triplets, so that the musicians reading the charts would "swing."
I guess you can see where this is going. The string players had no issues whatsoever reading the oddball notation. But the rhythm, brass, and reed sections, which were primarily if not entirely composed of jazz guys, had no idea what they were looking at.
It caused real problems at the sessions. But, of course, it all worked out in the end, because it is a great movie, and a great album, as long as you find the mono version and not the reprocessed-for-stereo version.