Ok, so I just found that Plex only supports specific models of Panasonic TVs. You can find a list in Which Smart TV models are supported? | Plex Support.
If your TV is not one of these supported models, then it will not have all the features of actual supported devices. This includes a bitrate limitation of 4 Mbps when transcoding is involved. That’s where this limitation is coming from. Transcoded streams do use thee DASH protocol so this limitation is due to using DASH. The only work around is to play a file that does not require transcoding.
HEVC should be direct playable, but not 10-bit files or HDR.
