Hello @casinoslotguy ,
Actually, I don't believe that is quite right, although you are correct in saying that switching to H.264 will fix the problem. It isn't correct, however, that the problem doesn't exist on 4K-capable Rokus. it does.
The problem appears to be a flat-out bug in Roku's playback of BT.709 colorspace video in MKV containers encoded with the X.265 (HEVC) codec. It affects all models that include the video upscaler feature. This means essentially all newer model Rokus, 4K or not.
It can be worked around by deleting the color info in the header of the MKV file, as described elsewhere in this thread, but in order for the workaround to be effective the file must be direct streamed. Guaranteeing that isn't easy, as also described elsewhere in this thread.