In other words - do not waste a slot on alliance with a nation that you are not planning to diplovassalize.
The whole idea about allies playing realpolitik is pretty much going back to 1.13 where a single province means that two nations will hate each other, except that now it can change without player even knowing what he did to upset the AI.
The reason you can't know that is because you didn't cause it. Others countries are making moves independently of the player, so there is an ongoing dynamic. You can work out how the AI is prioritizing and what it weighs (e.g. common enemies are weighed much higher than good relations, conflicting geopolitics like cross-cross rivalry tends to make it unstable, etc.). It's doable, but only if you lose the idea that the AI primarily does things in response to what the player does. If there was only one or a few AI countries near you, that's maybe how it would be, but in Europe, it's mostly independent of what players do, I think.