Briefly, it appears that the Roku improperly parses EDIDs from the downstream HDMI sink device and sends color-difference pixel formats (YCC-444 or YCC-420) despite the HDMI sink not supporting these pixel formats. The result is incorrect color (YCC-444) or incorrect color and quadrupled frames (YCC-420), because the sink is not capable of receiving YCC formats and interprets the pixel data as RGB.
After testing with a series of EDIDs, it appears that the Roku uses the presence of a CTA extension block in the EDID to determine whether or not a device supports YCC pixel formats: a single-block (DVI) EDID with only 1920x1080p60 present in the EDID resulted in 1080p60 without audio (compliant behaviour). However, using both v1.3 and v1.4 EDIDs with only 1080p60 and no support indicated for YCC pixel formats resulted in YCC-444 (non-compliant). EDIDs of both v1.3 and v1.4 for 4k60 and 4k30 with a CTA extension block indicating RGB-only support also showed similar behaviour (YCC-420 video, and in some cases the AVI infoframe indicated RGB while the Roku was sending color difference formats).
Additionally, I cannot find any details regarding RGB support online. While the HDMI 1.4b spec (section 6.2.3) says "All HDMI Sources and Sinks shall be capable of supporting RGB 4:4:4 pixel encoding." it's not clear how this relates to the maximum video format the device is capable of i.e. whether the spec requires the largest/highest frame rate mode to support RGB 4:4:4 or if only some intermediate modes must support RGB 4:4:4.
Regardless, this is causing issues with Roku's in installation environments where the downstream HDMI sink does not support color-difference pixel formats. Ideally, RGB would be supported at 4k60/4k30/1080p60, but at a bare minimum, support for RGB with audio at 1080p60 would be nice. Does anyone know of a way to get the Roku to follow the HDMI specification? Or is there something in the specifications that I'm missing?
Additionally, I tried all the above testing with HDCP disabled, 1.4 enabled, and 2.3 enabled, with no change in results.