I am not sure where I need to suggest bugs:
I have logged a few issues on the GitHub location (https://github.com/plantuml/plantuml), but it looks like most of the activity and resolution is in this forum?
At the risk of being chastised, let me try this forum for the same issues I logged there (and I'll continue using the medium that works):
The problem (bug) is that when I turn on shared target style for implementation (dotted lines), the dotted lines become solid:
skinparam groupInheritance 2
skinparam linetype poly
hide empty members
class B implements A
class C implements A
class Q extends P
class R extends P
class Concrete implements Abstract
The visualization from Concrete to Abstract correctly shows a dotted line.
The visualization from B and C into A show the shared target style, but the dotted lines are now solid...
Also note that the grouping does NOT work when using linetype ortho, which is another bug.
BUG1: Implementation dotted lines become solid when grouping.
BUG2: Grouping doesn't work with linetype ortho.