I have similar problems sending through a Rotel RSP1570 (HDMI 1.4a) to a Panasonic Plasma FHD.
The only thing that does this is JRiver. PowerDVD works, my hardware Panasonic Bluray players s work, the Set top box works, and JRiver has been working for years,
Until the last 6 months or so, and it is getting worse. Perhaps JRiver is moving on to HDMI 2.0 and above and dropping support for 1.4a??
At first is was only when playing 4k 10bit material, (the plasma does 10bit) but now appears to apply to almost everything.
I have tried both Dell and Intel Motherboards, and several RX 550 cards and several DP to HDMI cables where there is no HDMI on the Video card.
Tried several ATI / AMD drivers. I am pretty sure there is a problem between the AMD HD Audio device and JRiver.
Some times when in the advanced sound panel in widows, the ROTEL device disappears, which I have never seen before, and perhaps points at the now 15 or more years old Rotel.
It takes a few seconds for PowerDVD to get the sound going, so perhaps it is negotiating the channel??
Occasionally the JRiver does actually work.
This is probably not a common problem, so may not be a priority. Running W10 Pro I5. Will need help if any one wants traces etc.
Upgrading the Rotel would be a drag since everything else appears to play fine to it..?? Both WASAPI and DIRECTSOUND lockup after a few seconds of jerky video.
Suggestions welcome of course. Thanks