I am still having this issue and have been using PM5 for the last 10 months even though I purchased PM6. It is kind of a bummer that I can't use PM6.
I have continued testing on my side. It happens on my RAID and my SSD. One of the weird things that I noticed is that it is not picking up the initial keypress but, if I continue to hold the key, subsequent keystrokes are registered. For example, if I press right sometimes nothing happens but if I keep holding right, after that 1 second delay the images will start advancing. Really weird right? So it knows I pressed a key, it just doesn't register what I am telling it to do. For me, this only happens with arrow keys and my rating keys (1 and 0).
Does anyone have anything else that I can try? Is there a fix for this somewhere that I don't know about?
I do have one work-around. If I put the Navigator on one monitor and the Preview on my second monitor and then click on the image in the Navigator and use my left / right / 1 / 0 keys in that window - they don't get swallowed. You lose functionality like 'auto-advance' but you CAN use PM6 and it is SO much better than PM5. I hope this helps someone.