Audiobooks can have hundreds of tracks. For example this release has 998 tracks: https://musicbrainz.org/release/80d21196-929b-41f1-abe4-ea1b2d5ff2b6
If the number of tracks exceeds a limit, submitting AcoustIDs fails with this kind of error message:
In my experience with many audiobooks, the limit is around 270.
This behavior is reproducible on all releases where the number of tracks is above the limit.
This bug seems to be similar to https://tickets.metabrainz.org/browse/PICARD-1486