Hoping this is fixed soon as it is quite depressing to defeat a large army while knowing it has no meaningful impact in the war effort as the lords will just form a new army in a matter of days regardless of what you do.
Beta 1.3 :
The escape chance of prisoners is reduced by 25% when they are in mobile parties. It is reduced by 50% if they are at the player's settlement.
A bit better I guess...
About lords forming new armies in a matter of days :
They don't form armies in a matter of days, they spawn with free troops unfortunately