It DOES need fixing. If you have soldiers in neighbourhood should be irrelevant - enemy still should not escape through the lines.
It turns war into constant whack-a-mole situation.
Well, I'm not saying it's necessarily perfect, but mind the scale of things.
I'll questimate that most province to province borders fall between 10km (minor german state borders) to 100km (larger european province borders, we'll not even talk about other continents with this...)
So, to have single file of 1 man per meter, you need in smallest case 10000 men in combat, ok, now that sounds quite easy... then you check how often battles are fought in single line...
...yea, until you reach say, strenght 100 combats there is room even on smallest provinces to evade around the enemy lines, and even then it'd apply only to smallest provinces, really.
I feel almost cheating with keeping those 1 regiment backups behind the line, and especially circling the enemy, to prevent the retreats, and, yes, knowing that, you don't need to whack a mole in a war...
Now, I'm not really that much against change like that, but I really don't think it's necessary.