If the tracknumber is 0 (e.g. for a pregap track as on https://beta.musicbrainz.org/release/1efc5313-fa73-45b3-bc74-6e280bea4100) the tracknumber tag is not set.

See also discussion at https://community.metabrainz.org/t/tracknumber-for-track-0-not-being-set/485246