Sorry if this has already been reported (though I did look). There are a few variations on it:
Step 1: Open Dorico on first display screen
Step 2: Open Dorico mixer and drag to a second display screen.
Step 3: Touch a different app (a Browser lets say)
Outcome: Mixer disappears
Step 4: Click on Dorico
Outcome: Mixer re-appears but on the FIRST display, not the second one. It doesn’t matter which display Dorico is in; the mixer always re-appears on the first display.
This is really the same thing I think, but similar
Step 1: Open Dorico on first display
Step 2: Open Dorico mixer
Step 3: Click on the “e” button in the Play tab to open the VST for an instrument, or the "e’ to display an insert in the mixer
Outcome: Mixer disappears…
Same if I’m keeping a VST open for tweaking: The mixer disappears whenever I click on the VST window. I can see maybe if you’re trying to keep the screen clean or something, but the two screen example is the one I really miss: I often want to keep the mixer open on a second display.
I’m running Dorico 3.0.10, on a Mac Pro with Catalina.