Vorph
Silver Baronet of the Realm
That thread from the EU PS forums I mentioned also speculated that it was an HDCP handshaking issue. People said it never happens when you're using the YUV420 or YUV422 color space required for HDR (but the console always wants to go back to RGB on the home screen), or if you keep HDCP off (not practical at all). Some did say the cable swap fixed it, so I guess I'll wait and see what happens when I get a new one tomorrow before worrying about it anymore. Not sure why it seems specific to the Pro though, since people affected had all the major TV brands and several with Xbones said it never does it.