It appears that the Alliances tab while spectating doesn't show anything during the Alliance game type. I guess it only shows up on Dynamic Alliances? Or maybe it doesn't work properly while spectating? Need to test a little further. But, the Alliances tab on the spectator panel should show who's allied with who even on the Alliance game type sans dynamic alliances.
Just came to my attention that the game that I was running into this pre-dated the new Alliances tab. So maybe that's what was causing the issue.
