I tried using a few of Spitfire LABS free instruments with Dorico.
The MIDI keyboard does not respond (even though Dorico shows a small green light in the lower right corner on a key press).
Furthermore, one a LABS component is loaded the keyboard does not work with any subsequent instrument (in my case a VSL and an EastWest one).
Their AU works as expected within Garageband.
I already wrote their support team.