If you look at the replay you can look at it from one of the AI's point of view, go to the tab on relationships, and see where exactly all of the positive relationship is coming from.
That is a great idea and here is the data.
Be warned a lot of info.
Note I did not have a replay but had saved games at time points of 1:05, 1:15, 1:26 and 1:38. So I could reload the games and play as the TEC or VAsari to see what was going on.
The results are disturbing and it is pretty clear the devs need to tune their new diplomacy framework for the upper echelons of AI.
Some context first. This game had 3 AIs (all cruel) unlocked teams on a medium map. One vasari AI (pink), one Advent (green), and one TEC (red). My starting planets were hideous except one desert world right next to the TEC AI. I have no chance at a trade chain except via a starbase link and even then it is only 4 lanes. The TEC and I had been fighting since the get go. Pink was to my right but blocked by a pirate world. I did not see pink except for scout ships until the 1:26 time point via a wormhole right in the middle of my empire. I never saw green either until they died around the 90 minutes in.
Now the data:
1:05 timepoint:
Pink has an opinion of 3.60 with red and red has a 3.14 with pink. Pink is hitting green from one side and red is fighting me (and losing his shirt whenever he does) but is getting hit by green from behind. Pink for some reason doesn't like me (1.3) even though I have had no contact. In fact he is already offering red missions to hit me (wtf?).
So the breakdown: no trade yet, no envoys, no missions completed or rejected by either, military action approval is very small, racial biases still present, there is already a 2.0 AI relationship bonus (what is this?) and both have almost fully researched the diplomacy tree including all pacts up to level 7 so they share a 3.0 research bonus.
1:15 timepoint:
Red is focusing more of his task on defending his green flank. Pink is starting to hurt green on the other side. Pink still offers missions to red to blow up my structures (huh?). TEC is 5.4 with pink, pink has a 3.64 opinion of red TEC. Miliatry actions are in the area of 2.0 for TEC liking pink (probably over green getting hit). Btw I have researched envoys and the cultural bonsu ones but I have no place to send envoys to since red hates me and I can't get anywhere near the others.
So the breakdown: no trade yet, no mission bonuses, military approval is like 1.8 and 2.5 for the two mutually, racials still rpesent (this is the notable exception from research), AI relationship bonus is 2.0 and research bonus is 3.0 still.
1:26 timepoint:
Green has 4 worlds but is about to lose one and surrender (like 2 minutes later). Red TEC now has 7 envoys built. Pink like six. They are moving to each other's worlds to become mated for life. They have a trade alliance. Red has an opinion of 6.55 of pink and pink is 4.6. Military actions are 1.7 for pink seeing red and red has a 2.87 military action for pink (about the same as 1:15 timepoint). No ship vision yet. No planet vision yet. No pacts yet.
So the breakdown: Trade is ~0.25. Military action is 2.87 / 1.7. Envoy bonus is 0.69. AI relationship still 2.0 and research bonus is 3.0. Negative racials of -1.45 and -2.20 still in place.
Both are only offering missions to hit me. Yay!
Pink is sending a big fleet through the wormhole to hit my worlds.
Now it gets surreal.
1:38 timepoint:
Green died at 1:29.
Red and pink are both in the 1600-1800 fleet point range.
Suddenly they are bosom buddies with a cease fire. Red has 9 envoys and same for pink.
TEC likes pink at 17.6 and pink likes red TEC at 16.55 (WTF!!! in 12 minutes?)
Military actions are ~ 5.0 for TEC liking pink and pink is 4.4 for liking red's military actions (since they have both been pounding me).
Red hates me by this point since I have killed a ton of his ships.
Red and pink are attacking grav wells jointly.
So the breakdown: Still no missions completed or rejected by either party. AI relationship still 2.0. Research bonus is 3.0. Trade is 0.89. Military us 5.0 and 4.4 respectively. Racials still present and negative. But envoy bonus went from 0.69 to over 7 in 12 minutes. They have mutual planetary and ship vision. No resources or bribes ever given.
Summary:
The AI relationship bonus is overkill at upper AI levels where they max diplomacy in first 30-45 minutes.
Fleet bonuses or penalties were always fractional (<+/- 0.5).
That is a built in +5 relationship bonus at cruel and vicious AIs with no sweat between themselves.
The AI is offering missions against me when I haven't had any contact. In fact pink likes green more than me even though pink is hitting green hard.
I researched my cultural affinities around the 45 minute mark Total waste from what I can tell.
Red was foredoomed to hate me since we are right at each other's throats. But pink doesn't know me. Period. But they already dislike me enough to send missions out.
Missions between themselves were not a factor. No resources or credits sent or received on their sides. Military action was small and steady until green faltered.
Once green falls they move their mutual envoys around and bam it's over.
Their relationship jumps 8-9 points in 12 minutes.
They never even bothered to research away the racial modifier.
The aforementioned dynamics in medium maps are more brutal since you have three AIs and either two will team up after 3rd dies (one border, one from nowhere) or all three team up (two fronts simultaneous + one from nowhere).
Proposal:
At higher AIs the AI relationship of +2 is not a good thing. Remove it please. Sounds like a kluge fix from the past.
The cultural / diplomatic research bonus is unavoidable and will be +3 very early on for all AI between themselves.
The AI preferring AIs it is attacking and vice-versa over a human player it has no contact with by default (even if of same race) is poorly designed.
Maybe joint attacks on grav wells should not occur with cease fires (i.e. require an alliance) but that might be risky.
Then again the mass envoy deployment is a deal breaker probably.
Cruel and vicious is hard enough as is. The current diplomacy version as of 1.31 while ambitious is fatally flawed imo for smaller maps in its current form at cruel and vicious AIs.