[FireInMyHole]

Thanks, but made no difference.

Might sound stupid but test whatever build you're using with no other HDMI devices plugged into your TV (or AVR depending on your setup).  I had an issue were there was another HDMI device that was interfering with playback in Kodi (amongst other issues it caused) on my Fire TV Stick 4K but it didn't affect other streaming apps.  Once that device was removed then everything worked like normal including Kodi playback.  If you find that removing other HDMI devices improves the situation then slowing reconnect your other devices 1 at a time and test after each one until you find the device that is the culprit. 

You may find it makes no difference but sometimes you have to look at outside influences.  Could be something a simple as a bad power supply on another device (or the Fire TV Stick itself) causing issues or emitting electronic interference. 

Tried switching out the hardware - even a new stick. No go. Everything is out by 300 milliseconds now.

I could blame the TV, except MrMC running on the same stick decodes everything in sync in default settings. It's something related to Kodi on the Fire Stick 4K.