Yep, It looks like the same behavior can be seen in every new campaign and Western Empire gets wrecked really fast.
Adding a "diplomatic cooldown" of a few days of interactions between factions would address this particular issue, though it's a lazy way to do it... Would rather see a more robust implementation actually emulating some kind of consideration and acting on that, rather than be driven by placeholder if/else clauses.
Aside from fixing the Aserai making peace in the first day, I think that something else should be done to avoid some factions fighting against two enemies at same time getting wrecked in few days. To be honest, not sure about what could be done but we should have something in order to avoid snowballing and factions getting destroyed in few days.
Maybe some kind of system which allows factions to play totally defensively when they are outnumbered by two enemies or something else.
Flip the defender's campaign AI behavior to focus solely on defense as long as there is large enemy presence in "their" lands. Implement AI suing for peace (with a cooldown period before the aggressor can again declare war on them) with one of the factions, at the expense of their finances, so they can focus on a single enemy. This is where villages as independent fiefs would've been really helpful as part of the bargaining process, so the defending faction could, at the cost of a fief, at least try to secure peace with one of the aggressors, but hey - Bannerlord "improvement" over Warband.
At least having "sue for peace" coding based on money alone (uugh, but whatever - castles and towns are too valuable to trade away that way...) would prevent a faction from being immediately ganked - at least as long as its ruler had money to pay off for temporary truce.
This should all (aside from switching to "defensive AI") come after doing some basic calculation of enemy forces, so two factions with comparable number of troops (for whatever reason) to the defender should be allowed.
tl;dr version: "Sue for peace" implementation based on money, "cooldown" timer in between diplomatic exchanges (at least the SAME ones), and in general "mods will fix it"