3D on 2016 LG OLED is indeed stunning ... I had Samsung D8000 before, 3D was terrible there (probably what killed 3D as Samsung had more TV sales) ... but that's a separate discussion
SO it does seems your trouble is same as mine ... I will post steps to remedy it ... see
https://yabb.jriver.com/interact/index.php/topic,113941.0.html for full discussion about it
reset your MadVR setting to default ... then madVR setting these are bare bone necessary setting to get it working:
[rendering -> stereo 3d]
enable stereo 3d playback: check
when playing 2D content: disable os stereo 3d support for all displays
when playing 3d content: enable
retore os stereo 3d setting when media player is closed: uncheck (dont think this part matter ... just personal preference)
create profile for [rendering -> general setting]
one for 3D ... check enable automatic fullscreen exclusive mode and delay playback start until rendere queue is full
one for 2D ... check use Direct3D 11/present frame for every VSync and delay playback start until rendere queue is full
my 3D files have extension mk3d ... so profile switching logic is simple
if (fileExt = "mk3d") "name of 3D profile" else "name of 2D profile"
... now the hard part (this had to be done everytime windows restart)
(1)in nvidia control panel(cpl) i set 1080p23 with 3d off
in windows cpl 3d off too (hdr off too)
(2)close both cpl and then restart computer ... once restarted i wait till everything loads (not sure if thats necessary)
then i open both windows and nvidia cpl
in windows cpl, turn on 3D ... nvidia cpl will automatically on 3D too
(3) then in nvidia cpl change resolution to 4k 60Hz 32 bit RGB 8bpc fulldynamic
(4) then in windows cpl turn 3D off ... nvidia cpl will automatically update 3D off
(5) then in windows cpl turn 3D on... nvidia cpl will automatically update 3D on ...
now 3D setting is link between NVIDIA and Windows ... so madVR can switch it
Final step ... if i use 4K resolution for windows and turn TV off ... that 3D link gets broken ... if yours does same, then only option is to run Windows in 1080p23 or 1080p24 ... so your MC will be in HD ... but when playing movies it will switch to 4K ... not a too terrible trade off to get it working
Let me know how it goes