While in Play mode, engaging the drop-down menu to choose a different flow and then pressing space bar results in a screen/program freeze. To get out of the freeze, switch to a different program (like a web browser, etc.) and back to Dorico.
I expect the problem is merely that the focus is stuck on the flow selector drop-down: clicking somewhere else, e.g. on any of the tracks below the drop-down, should restore the focus such that Space then starts playback rather than re-opens the flow selector.