Plexamp is using the same artist sorting as all the other Plex apps, if you pick “Title” as the sorting criterium.
You can see that “Various Artists” are at first position in this screenshot, because I edited the Artist Sort title in Plex to achieve that:
Plex doesn’t use the Sort Title from Musicbrainz directly.
These deviant sortings are coming from your files. (They might have been embedded by Musicbrainz Picard though.)
If any of them have an embedded “Artist Sort” meta tag, Plex is using its content.
If you don’t want that, verify that none of your songs has this “Artist Sort” meta tag.
You will have to edit the affected artists and make sure that the contents of the Artist title and the Artist Sort title are identical.
(You might wanna tell Musicbrainz Picard to not embed the ArtistSort tag in general. See [HowTo] Music Organizing with MusicBrainz Picard - #6 by OttoKerner for instructions.)
