Tag dump sees a tag ALBUM ARTIST SORT but import from tags does not import it into the library field ALBUM ARTIST SORT. Assuming there are no extraneous special characters luring in those names, there is an inconsistency in how MC is handling this tag/field.
The id3v2 and vorbis names for ALBUM ARTIST SORT are TSO2 and ALBUMARTISTSORT
My guess is that when MC added COMPOSER SORT and ALBUM ARTIST SORT the mappings between tags and library field names and display names became inconsistent in their handling of spaces. Before those changes, MC was not doing any mapping of them. The fact that your ALBUMARTISTSORT tag imports into the ALBUM ARTIST SORT library field supports that thinking.
For a flac file, which uses vorbis, you might also try creating a library field of ALBUMARTISTSORT and see if that populates from the ALBUM ARTIST SORT tag in the file.
It would be best to use a hex editor and go into the file and see what is really stored there. Then it would be easier to see what MC is recognizing and how it is mapping it to a library field and to a display name.
EDIT: I do not see a MC defined [Composer Sort] or [Album Artist Sort] tag in manage library fields. It seems like a mapping of [Album Artist Sort] to TS02 or albumartistsort may have been added per Rod's link, but no actual library field by that name was added. So, if a user defines a field of [Album Artist Sort] it automatically gets mapped to albumartistsort in the vorbis tag in the file, even though the user has no way of knowing that. It seems like that makes it impossible to link a library field to a file tag of ALBUM ARTIST SORT, unless there is some MC magic that forces a library field to be tied to a specific file tag of a different name. This basically orphans previous file tag values of ALBUM ARTIST SORT. To make them visible, I guess they would have to be changed to a file tag name of albumartistsort in another program to map them to a library field of Album Artist Sort.
Still a little mysterious to me.