I believe that Style/Specific_types_of_releases/Soundtrack's intent (as demonstrated by the choice of release group examples) is that text like "Original Motion Picture Score" or "Music from the Motion Picture" should be formatted as subtitles.

This interpretation seems to be broadly accepted in the community forum, too:

• https://community.metabrainz.org/t/original-motion-picture-soundtrack-subtitle-or-eti/607814
• https://community.metabrainz.org/t/release-title-for-soundtrack/551816
• https://community.metabrainz.org/t/release-name-for-soundtrack/496508

If my understanding is correct, I'd like to add a sentence to the release title section of the soundtrack guidelines to make this explicit rather than just implied. Does something like the following seem okay?

Text such as "Original Motion Picture Score" or "Music from the Motion Picture" should be formatted as a subtitle.

I would also recommend adding one or two examples where a film title itself contains a subtitle. These come from https://community.metabrainz.org/t/release-name-for-soundtrack/496508/15:

• 1492: Conquest of Paradise: Music From the Original Soundtrack
• 2001: A Space Odyssey: Original Motion Picture Soundtrack