This seems to be a problem with the instrument definition in Dorico, not than the playback libraries.
For concert pitch, the bass trumpets play an octave below the written pitch. Whether that is “correct” might be a matter of opinion, but the difference between transposed and concert pitch goes to the wrong octave for the Eb bass, but is correct for the Bb bass.
So IMO the “logical” place to work round this is with a clef change using the octave transposition property, not by tweaking the expression maps or sample players.
FWIW having recently input some Wagner that included bass trumpet in Eb, I didn’t notice that the playback pitch was in the wrong octave. Some more research of Wagner’s (often weird) ideas about brass notation might be called for…