It looks like this problem has been well-known for a few years now, but no firmware fix from Roku to keep from trying to Dolby Vision transcode rec 709 material in MKV containers when being sent to 1080p display devices.
I reached out to online chat and phone support yesterday, but spent two hours having people tell me to reset my Roku and try again - beyond worthless.
It appears OS 13 has finally solved this issue. Can you verify?
No, it doesn't appear fixed. If I force direct stream on my Plex client, the color is interpreted wrong on the Roku. If I let Plex transcode (which my NAS is not fast enough to do reliably for a 4k stream), it displays the correct color.
Obviously my preference at home, and for almost all of my viewing, is to direct stream rather than be forced to transcode to see correct colors.