This question has come up a few times over the past couple of months. While there's always more to say, you might like to check out this detailed discussion of the issue led by u/sunagainstgold:

What ideas did "Westphalian sovereignty" replace?

And I commented on the shortcomings of the portrayal of the treaty in International Relations theory here:

Why is the treaty of Westphalia significant?

Other older threads covering much the same ground can be seen here and here.