The first thing one should probably ask: Can't one get rid of genre specific rules at all?
Classifying music intro genres is highly subjective and in many cases their either is no answer at all or even multiple may apply.
Who'd give the definite answer? The majority of people? The artist? The music scientist who analysed a specific genre for 40 years?
In the case of MB it probably often ends up in depending on who's actually around for voting making the whole thing extremely random and ambiguous.
The second question is probably: Is soundtrack a genre? Perhaps not "soundtrack" in the strictest definition, but I'd call "film music" a genre by it's own.
The genre often depends less on the instruments used to play it but the style of the music and even though this is probably much broader in film music than any other genre, film music has still some general elements, that makes it IMHO distinct from other genres. E.g. the way emotions and story is transported, the association of persons/groups/etc. with specific themes and leitmotifs.
reosarevok said "A recording of classical arrangements by a rock band" above.
I think that's a quite difficult questions... and I wouldn't generally agree with that. "Classical arrangement" means in many cases "same music, played by an orchestra, or with typical classical music instruments, like violins, trumpets, etc."
If we'd go by that, we essentially left classification by genres and make classification by instruments, which is IMHO even more stupid than classification be genres.
Why should the same music played on a e-guitar result in different tagging rules than when played on a violin?
Also, there is no static and definite definition on which instruments relate to which genre or e.g. which instruments belong to orchestras.
Sure there are the typical suspects,... but I doubt one could call anything played on a saxophone Jazz... neither is everything played on a piano classical (or would you call Meat Loaf's I'd do anything for love classical, because of the intro)?
Another problem is that the current definition of classical music is far to broad, allowing basically everything to be classical if one can just identify enough classical music instruments in it.
E.g. I'd never consider medieval music classical music... it's completely different in it's style.
So my preferred solution would be simply to do away with genre-based rules at all. The whole concept is IMHO nearly as stupid and problematic as our ever ongoing conflict between the de-facto use of release-countries and their de-jure semantics (and the whole concept of release events at all).
It would be IMHO better, if one would have e.g. special rules based on the "content" of music,... e.g. a rule that says: if you have a piece of music, that is a re-interpretation of some other work (but not so distinct to count as a new cover-version), you're allowed to place in the (re-interpreting) performer as artist (in addition to e.g. the composer), because that one is probably the one/group who the people having this release are interested in.
I'd hope that such rules are more clear when to apply than e.g. the question soundtrack/classical.
If all that fails, I'd say the next best thing would be to define what should be done in case of a conflict.
E.g. when two parties make edits and assume a different genre, and if they cannot agree,... the normal non-genre-specific rules shall apply.
Of course that makes the whole thing again a matter of chance, as it depends who's voting and whether the conflict pops even up or not.
If I'd be asked for a definition of soundtrack/film music/scores I'd probably say the following:
1) original music of a film, series or any other moving picture as well as of (e.g. computer) games, if:
[original in the sense of either used "as is" in the film/series/etc. or officially released by the filmmakers for that piece; this would e.g. also mean that a song like Starman as used in the movie The Martian, would still allow for a soundtrack, if it's part of the movie soundtrack album, but just the song alone or on any album without any further relation to the film, would not qualify it for soundtrack]
- the moving picture is not a (e.g. live) recording of a normal (i.e. not intended for film only) production of a musical, theatre, opera or any similar play
2) music directly made for such film, even if not used in it
[which would allow for any bonus tracks, rejected soundtracks, unused cues, album versions of film music, etc. to count as such]
3) Music based on film music, if it's either directly based (e.g. re-interpretations) or if still a majority(!) of the work can be directly(!) related to the film music.
[that would IMHO mean, that re-interpretations like the ones from The City of Prague Orchestra are still film music,... also if John William plays Star Wars with any other orchestra, it would be still film music.... but e.g. Adele's Sykfall wouldn't bet film music, just because the first few seconds contain parts of the Jame Bond theme,... neither would e.g. such music that everyone more or less remembers to a very famous piece of film music, but which was changed enough in order not having to pay license fees count as film music... take e.g. the first 10 seconds of https://www.youtube.com/watch?v=1w8Z0UOXVaY ... it remembers to Star Wars, but it's not really the music, and shouldn't count as film music IMHO).