Since Dorico 2.0 exporting particular selected flows to audio does export the flow, but also adds a lot of silence equal to the duration other flows, requiring subsequent trimming (on a Mac in Quicktime). I think this only happens with flows subsequent to the one(s) selected but I’m not sure. I’m using NotePerformer and haven’t tried it with Halion or anything else.
Yes, we’re aware of this problem (it’s caused by the way Dorico has to assemble a project timeline to accommodate the video features). Paul has been reworking this area since Dorico 2.0’s release so this will be in better shape in the next release. Sorry for the inconvenience in the meantime.