Same recording, multiple releases. Some have relation-list, some don't

I’m looking at a recording that is included in a number of box set compilations. For some of these releases, the API attaches relationships via the <relation-list> XML entity. For others, this list does not appear.

Using the web interface, all the releases seem to include relationships (orchestra, conductor, work). Yet for some, the API does not return them. What’s missing?

Here are the specifics. In all releases, we’re talking about track 1 of CD 1, that is, the first track on the release.

Recording: Recording “Symphony in D major, Hob I:1: I. Presto” by Austro-Hungarian Haydn Orchestra, Ádám Fischer - MusicBrainz

This release includes the <relation-list>:

These releases do not:

In human-readable terms, the track on the pseudo-release “Haydn Edition (discs 1 - 10)” contains <relation-list>; the tracks on two box sets named “Haydn Edition” do not.

Thanks for any pointers. I’m just starting to learn the API, so I may have missed something obvious.

See here: