I have a similar technical problem as the OP.
Summary of problem:
Thanks, in advance for your attention, Naughty Dog!
Try the following:
That seems to have worked. Thank you!
BTW, Siege is not a selectable game type, correct? It will pop up in Cooperative Arena at some point?
That is correct. If you're in a party of two or three, you can select Siege in custom games. But in matchmaking, it will be part of co-op arena as you said.