I don't know if anyone else has had this problem, but I just finished a game, but not exactly.
I defeated all the opponents through the destroying their homeworlds.  Their factions were officially defeated, but the victory screen did not appear.  Furthermore, my ally ended all of our pacts, but did NOT set me to enemy.  The timer did not appear on the diplomacy screen for to declare war.  Even after several minutes, they still did not declare war or even break any treaties (Still had ship vision, etc.  Only cancelled our pacts)
One further bit:  On the exit game screen with the stats, upon trying to return to the main menu, my game froze (probably unrelated and not a major problem)
I'll list the exact details of what occurred to see if this is a general problem or rare.

- 2v2 With locked teams, normal start, all normal rates except fast income

- No allied victory, but the Capital Victory option enabled (all other options disabled)

- Defeated both enemies by destroying their capital

- Played on Razor's Edge

- I was rebels, ally was loyalists, facing a rebels and a loyalists

I can upload a save if desired.