My favorite campaign, so sad.
Essentially the bug is that when you play as Caesar, and you conquer your first settlement, after that point your armies can barely move. Like their movement range is reduced to 10% for the rest of the game and it makes the campaign unplayable.

Any idea how to fix this?

I'm not using submods and this bug has persisted for over a year now, there's others who have posted about it on DeI Twcenter and got no solution, so I'm worried whether it's actually possible to fix or not?