Prior to updating to Dorico 5.1.50 I had occasionally noticed this issue, but since the update it seems to have become much more pronounced.
After selecting and applying a Playback Template which includes HSO (any one of those highlighted in the four attached screenshots), inputting a note using either a MIDI keyboard or the computer keyboard and selecting an existing note using the mouse or arrow keys produces a very low volume level for the note. Playback volume level is as one would expect and follows any dynamic markings. The low volume level issue appears to be independent of dynamic markings - I tried from ppp to fff and the volume did not change when selecting a note or inputting it as described earlier.
Playback Templates which do not include HSO behave normally. It is only those which include HSO which have the problem.
Has anyone else experienced or noticed this? I have tried resetting the various options and settings which seem related (Reset Playback Overrides, Reset Default Output Level, Use fixed volume to play selected notes, checking out Stage and Space Templates, mixer settings, etc.), re-launched Dorico, and restarted the computer. After a fairly exhaustive investigation, the only common feature is the choice of Playback Template.
Does anyone have any ideas as to what the underlying cause might be?
Edit (19 hours later):
To see if it made any difference, I used Steinberg Download Assistant to re-install HALion Symphonic Orchestra. There was no change in behaviour. Until a solution is found, either by myself or by others with more insight into how these things work, I will use Playback Templates which do not include HSO.