In short… Save… Close… Restart game & load save… (Possibly restart PC, to reset cache). Hopefully things will get better after that….
Otherwise as already stated, start a new campaign and it usually does the trick. Some campaigns turn out like that - others don’t. And so it goes (including mods I would imagine). Personally I would not categorize these circumstances as a bug, if it is, then it is a small one compared with others in MTW. Anyhow, alliances all over won’t last forever – that’s my experience.
Here is another thought, if you guys don’t like peace - do consider going to war on your own…
You will probably get some dirty mayhem in no time…