I don't know if this is expected or a bug, but the manual doesn't mention anything about this issue and I can't say that I've seen it before.
With an sACN Output Viewer tab open, selecting any channel will freeze the entire universe it's a part of. The universe will remain frozen until another channel is selected or the sACN Output Viewer tab is closed. This output freeze is not reflected in the viewer, though; if (for example) I think the network has died and start randomly selecting fixtures and moving them around to see why they aren't moving, the Output Viewer will show all of that data changing, but the universe will remain frozen.
This is on a DVI 4K Gio running 2.8.3