I did see somewhat similar results on my M70-C3 (the equivalent 2015 model) with HDMI 1-4 displaying much better compatibility than 5, though they of course gave higher lag. However, HDMI 5 hasn’t had any issues with 3x since several OSSC firmwares ago, while 5x has consistently never worked on that port. I also have a PS4 Pro, but I’ve never seen any kind of handshake issues like you describe.
The one odd thing I did notice for my first couple years of using this TV was that the video would occasionally drop for a second with 4K sources (I know it was the TV and not the source, because my sound system kept playing the audio properly). That’s cleared up in the past six months or so, though, possibly due to a firmware update for the TV.
Don’t know how much that helps, but I figure a little more info can’t hurt.