Using a MIDI keyboard, I entered the (treble clef) notes in the picture into horns of various pitches. I made sure that Edit > Transposed Pitch was selected before doing so.
For Horns in C (alto), B♭ (alto) and G (alto), Horns in F, E♭, D and D♭, the notated pitch stayed the same as I had played in, and the sound played corresponded to the pitch of the horn.
However, as I continued choosing horns of lower pitch (and playing exactly the same notes on the MIDI keyboard), the notation and the sound played jumped to an octave higher. Those which displayed this behaviour were Horn in C (sounds 8va bassa), Horn in B (sounds 8va bassa), Horn (basso) in B♭ and Horn (basso) in A.
Is this intended behaviour?
I found that to achieve the notation in the correct octave, I had to play the notes an octave lower ie in the actual sounding octave.
To the best of my knowledge and experience, when reading from the same notes, a Horn in B♭ basso should sound in the same octave as baritone horn (written in treble clef), tenor saxophone, euphonium and bass clarinet, namely a major 9th lower than written.
I think the playback of the displayed notation is in the correct octave. It just seems to be how Dorico interprets what is played in for the particular instrument - in this case, the low horns.