GT1030 spontaneously refuses to go into Full color on HDMI.

I haven’t been able to track what has changed. The display will resync after change from Limited to Full and vice versa with nvidia-settings, but the output is always in Limited mode. This was working properly (set to Full) for about 6 months. Powercycling all equipment was ineffective. Forcing the setting in xorg.conf is also ineffective. What else can I check?

Ubuntu 16.04
nvidia-387 and nvidia-390
4.4.0-112-generic and 4.4.0-116-generic and
GT 1030

HDMI-0 connected 3840x2160+0+0 (0x1c9) normal (normal left inverted right x axis y axis) 1600mm x 900mm
Identifier: 0x1bd
Timestamp: 40311
Subpixel: unknown
Gamma: 1.0:1.0:1.0
Brightness: 1.0
Clones:
CRTC: 0
CRTCs: 0 1
Transform: 1.000000 0.000000 0.000000
0.000000 1.000000 0.000000
0.000000 0.000000 1.000000
filter:
CscMatrix: 65536 0 0 0 0 65536 0 0 0 0 65536 0
EDID:
00ffffffffffff0011ee4e0001010101
001a010380a05a780aee91a3544c9926
0f5054a1080031404540614071408180
01010101010108e80030f2705a80b058
8a0040846300001e023a801871382d40
582c450040846300001e000000fc0044
454e4f4e2d41565248440a20000000fd
003a791e883c000a20202020202001ab
020372f15f6160101f66650413051403
021220212215015d5e5f6263643f400e
230f241e380f7f073d1ec01507505f7e
03570601677e014d02005f7e01835f00
006e030c002200b83c20008001020304
67d85dc401788003e20f33e3060d01eb
0146d000260a0975805b6ce305c000e2
00cf00000000000000000000000000ef
BorderDimensions: 4
supported: 4
Border: 0 0 0 0
range: (0, 65535)
SignalFormat: TMDS
supported: TMDS
ConnectorType: HDMI
ConnectorNumber: 1
_ConnectorLocation: 1

update

I was able to get the range control to show some functionality by plugging direct into my LG TV, versus through my Denon 4300 AVR. However, it doesn’t work again when connected directly the AVR, even trying unused HDMI inputs.

So I tried copying the EDID directly from the LG and then using that as a custom EDID when connected to the Denon AVR. This unfortunately has no effect on the color range control.

So it seems overnight the Nvidia GT 1030 became allergic to the HDMI input on the Denon, without any apparent changes to software??? There must be a way to force the Nvidia card to the correct setting?

Check if video conversion/scaling is turned on on the Denon and turn it off.