The background of the group/end container should be transparent. The "new" behavior of plantuml.2017.08.jar is not consistent with the previous versions. The defect impacts much more large diagrams where there are many groups defined or a group container overlays almost the whole sequence. The "new" behavior worsens diagram readability from coloring perspective.
This should be either fixed or there should be a way to configure the "old" behavior within @plantuml definition
box "A" #DD55DD
box "B" #55CCDD
user->device: do something
group if condition 1