You need to get the “Composed by” section displayed for all your composers, not just for Classical composers. So as a workaround, you can set a composer as a “Classical” composer in the Artist editor on the Composer page:
That will then display the “Composed by” section on the composer’s page in the Discography list:
This will just display those albums in your library that have pieces composed by the selected composer rather than all the albums known to Roon that meet this criterion, but I think that is what you are wanting…