Just to answer to an old unanswered question (and confirm that is now OK);
We can now observe the experted result:
A -[thickness=10]> B : some_labels
a-[thickness=5]>b: some text
b-[#red,dashed,thickness=10]>d: some other text
[See on PlantUML online server]
If that can help,
I just updated the documentation with that.