ddcctl doesn't work on new Mac Pro 10.15.6 with BenQ EL2870

I've just cloned the latest git release and compiled with make intel for a new Mac Pro running 10.15.6 Catalina. The monitor is a BenQ EL2870 and this can be controlled with a MacBook Pro running High Sierra, so it is definitely DDC enabled. I have also used this monitor with a Mac Mini running Catalina, so maybe the issue is with the graphics card? These are the graphics/display details from System Information:

AMD Radeon Pro 580X:

  Chipset Model:    AMD Radeon Pro 580X
  Type: GPU
  Bus:  PCIe
  Slot: Slot-1
  PCIe Lane Width:  x16
  VRAM (Total): 8 GB
  Vendor:   AMD (0x1002)
  Device ID:    0x67df
  Revision ID:  0x00c0
  ROM Revision: 113-C9440C-171
  VBIOS Version:    113-C944A1XT-014
  EFI Driver Version:   01.01.183
  Metal:    Supported, feature set macOS GPUFamily2 v1
  Displays:
BenQ EL2870U:
  Resolution:   6720 x 3780
  UI Looks like:    3360 x 1890 @ 60 Hz
  Framebuffer Depth:    30-Bit Colour (ARGB2101010)
  Display Serial Number:    YBJ01334SL0 
  Main Display: Yes
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Adapter Type: DVI or HDMI
  Automatically Adjust Brightness:  No
  Adapter Firmware Version: 2.19

The ddcctl command runs, but no settings on the monitor are changed, for example:

./ddcctl -d 1 -b 10 
D: NSScreen #1952318984 (3360x1890 0°) HiDPI
I: found 1 external display
I: polling display 1's EDID
I: got edid.serial: YBJ01334SL0
I: got edid.name: BenQ EL2870U
D: action: b: 10
D: setting VCP control #16 => 10

kfix / ddcctl

ddcctl doesn't work on new Mac Pro 10.15.6 with BenQ EL2870 #82