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
This iissue is the same as the one on the thread. https://community.etcconnect.com/control_consoles/f/eos-family/27425/poss-bug-2-7-2-sacn-viewer/90101#90101
I posted a long comment explaining why multicast can behave this way if the switches are being too clever. You are probably seeing that ikind of issue. There's a kind of contradiction with multicast in that you actually want it mostly broadcast across the switches to any device that wants it but to get most of the benefits of multicast over broadcast you dont want that happening. The switches try to work out who is subscrobed to the multicast and only propagate it to their segments. When the viewer subscribes it seems some switches then decide the other subscribers dont need it. anymore (fuller details on that post). Its probably more likely to occur if there are two switches between the desk and the ultimate subscriber (the rig).