Situation begins to be complex here :-)
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.
Let's talk about your first case :
Group node should not be transparent
(as in **v1.2020.10**, where Group node is white)
Well, I think v1.2020.10 is defect : there is no reason why Group node should be white. Here, it is transparent, because somehow it inherits the general "backgroundColor" setting. Does it make sense ?