As @Matze0211 mentioned, this is something out of the skill control. Basically, only a handful of display categories such as lights are supported part of the Alexa-enabled group feature. Unfortunately, blinds aren’t part of them as of yet. However, you can use the workaround below to accomplish what you want:
As a side note, I noticed that you are using the RangeValue attribute to control your shutter. Any reason why you don’t want to use the default PositionState attribute for Shutter?
