Hi, sorry for the late reply, I was hoping another admin might have had your answer.
I haven't come across this problem myself and I can't suggest a solution.
However, you could post on the community help page and someone in the world might know?
If i understand your question I don't know of a way to "fix" it.
That is to say I'm not sure if anythings broken. If you're zooming in the lines may look a bit softer if you're using the non-vector line tools.
Or line tools with soft edges.
But If you use the vector line tools, then the lines should be clean no matter the zoom.
the other issue that comes to mind is that it might be your computers render speed.
to check it see If when you change the zoom and then -Wait a Bit- do the lines become sharper? or stay blurry?
Other than that I'm not sure.
you might want to post the question to the guy in the video below.
https://www.youtube.com/watch?v=tKzKAZJ7bRM
peace
Avery