Dorico uses Expression Maps (multiple of which can form Playback Templates) to interface between Dorico and the relevant VST. Many VSTs use low notes as key switches to tell the VST how to switch between different patches, e.g. one low note could be used for a sustain patch, another for a staccato, another from tremolo etc. This is convenient for live performance of VSTs from a MIDI keyboard, as the left hand can change the sound while the right hand plays the melody.
Expression Maps allow Dorico to do this patch switching automatically when “reading” your music.
However each VST manufacturer uses different keyswitches. If you’re using an instrument that uses one Expression Map, you can’t expect the same Expression Map to work with a different instrument from the same manufacturer, let alone another completely different instrument from another manufacturer. You need to also change the Expression Map.
The low drone that you’re hearing is the result of Dorico sending a key switch to an instrument that can’t understand that key switch; the instrument is then playing it as an audible note.
Hopefully this explanation gives you some guidance as to what to read up on in the manual.
I’ve taken the liberty of editing the title of this thread, as
a) it was unspecific about the actual issue and
b) this isn’t a bug; it’s a lack of understanding on your part