You will probably have to apply what I suggested in the importing application (Sibelius, MuseScore, Finale, etc.) if it offers that feature when importing MIDI.
Because these low notes, which apparently are caused by MIDI keyswitches, seem to affect only strings (I might be wrong there), you could try changing the instruments to something else before exporting MIDI from Dorico.
Also (and a much better answer), see the solution given by Daniel Spreadbury which is no. 5 in this thread:
Those notes are all key switches, so they’re certainly not vestigial. When you export MIDI from Dorico, it exports exactly the same music that it plays back. If key switches are used during playback, then key switches will appear in the exported MIDI.
If you want to ensure that your exported MIDI data contains only notes, the best thing to do is apply the HSSE (SE) playback template before you export your file, as this playback template uses only simple GM-style sounds that don’t cause any key switches or MIDI CC values to be played back. If you’ve tweaked the playback of your project, I suggest you change the playback template on a copy of the project rather than on your original, as changing playback template is not undoable.