Doesn't work. I created a custom Field and set it's expression to [AlbumArtist]. And that's exactly what shows up, that text exactly in every album. If I change the expression to [Album Artist], then a blank value show up as it should, with there being no tag Album Artist
You have to type the whole expression. In a pane view, right click on a column heading and choose Add Expression Column... Give the column an appropriate name (Albumartist, for example) and paste the expression in the expression box. The expression should be:
If(IsEmpty([Albumartist]),[album artist (auto)],[Albumartist])
Copy and paste to get it right.
Are you sure you have created a MC database field named ALBUMARTIST (Tools=>Options=>Library & Folders=>Manage Library Fields=>Add New Field)? If "[Albumartist]" is shown when you in an expression type only [Albumartist], I think that indicates that there is no such MC database field.
If you create a new MC database field I think you will have to rescan the existing files in your MC library (File=>Library=>Import) to get the field populated. (When you add new files to your collection this is done automatically, so you will have to do this just once)
Have you been using dBpoweramp for a long time and updated? A couple of years ago dBpoweramp changed the naming of the album artist tag (Vorbis comment) from ALBUM ARTIST to ALBUMARTIST in flac files (but I think the tag editor showed the same name as before). In case you have some files with ALBUMARTIST and some with ALBUM ARTIST you could in MC create database fields for both ALBUMARTIST and ALBUM ARTIST, and also for COMPILATION, and use this expression (provided you have only flac files ripped with dBpoweramp):
If(IsEqual([Compilation],1),/(Various Artists/),If(IsEmpty([Albumartist]),[album artist)],[Albumartist]))
I would, however, in that case recommend retagging so that you have only either ALBUMARTIST or ALBUM ARTIST in your files.