As mentionned on /11657 :
First, the "pale line on the bottom of node" is due to shadowing.
I agree that it's an issue, but I won't talk about it here (because it's already complex enough :-).
You may create a new question about this specific point if you wish.
Here is a specific defect for that.
On version 2020.11, on PNG export, for node group, when use of 'Backgroundcolor transparent', we observe "Pale line on the bottom and right of node":
[Click to see on PlantUML server]
Example, with 'navy' blue color, in order to good seeing this issue.
We don't observe that on SVG export.
Thanks for your work,