According to the documentation, when a group contains some flagged items, the group should be marked with a hyphen to indicate an indeterminate state. However, in v4.0 Beta 2, this doesn’t seem to be happening. I don’t have any indeterminate flags.
I’m not sure if it’s a bug in the beta or intentional. Development will have to comment on this.
It’s indeed a bug, the next release will fix this.