It might be helpful to know that there’s a difference—in beets and most other tools—between the abstract field name that you see as a user and the on-disk name used in tags. For example, FLAC files use a field whose “physical name” is `DISCC`, but that’s not too helpful for humans—so beets and other tools use a “logical name” like `disctotal`.
It’s the same way for `ORIGINALDATE`. Most tools will show you a more legible name, like “Original Year,” or `original_year` in beets’ case, that even splits the part of the date apart. But that doesn’t mean metaflac will show you that name!
It might be worth doing a little bit of research into Picard and Foobar to see how they interpret this data. I did a little bit of looking, and Picard’s docs show that they use both `ORIGINALDATE` and `ORIGINALYEAR`:
https://picard.musicbrainz.org/docs/mappings/#cite_note-0
I’m not sure what it means that both are present in the list. In any case, if you can nail down exactly what those other tools are looking for in their “physical” tag storage format, please do file a bug—we can make sure we’re compatible.
Adrian