Alright, finally got it to work for my 2016 LG E6 and I expect it to work for all other 2016 OLED models as well. Changes I needed -
Shield Pro > Device Preferences > Display & Sound > Resolution: 4K @23.976Hz / Dolby Vision and HDR10 Ready
Plex > Settings > Advanced > Refresh Rate Switching: OFF
#1 was mandatory for me. I think Plex probes whether the current profile is “DV ready” and gets a response “no” if Shield is set to only “HDR10 Ready”. With resolution set to 4K HDR10, I never got to play any DV material (24Hz or 60Hz) in DV.
With #1 set as above and #2 set to ON, it was a hit or miss for me (could be a bug in Plex). Some 24Hz content would play but many wouldn’t. With #2 set to OFF, DV always turns on for DV material. Obviously 60Hz material is not smooth when forced to play at 24Hz but the TV can’t do DV @60Hz anyway so have to live with that if DV is a must.
[Possible enhancement: I don’t know if it’s possible for Plex (on Android TV platform) to detect if a DV profile is available with the given player / TV combination even if the device is not set to “DV ready” profile. If possible and there is a refresh rate matching DV profile, the devs might want to play the material in DV (assuming Android TV allows it) instead of HDR10 as most people will likely want that. Or maybe even have an option “Prioritize DV over HDR10 when available: ON|OFF”.]
Hope this helps someone who has a 2016 OLED and is late to the DV party like me.