xrandr refuses attempt to addmode

Question

Hi I have a monitor that doesn't seem to report all the video modes it's capable of. I have been able to trick my display system (in Ubuntu 22.04) into believing it's capable of these modes by adding an HDMI splitter and putting a second display on that. When I do that, the second display reports additional, modes, xrandr sees and lists these modes. The two modes I want to use are now included in the list and I can select them (either with xrandr or with the Ubuntu "settings" tool). When I select either of them the problemaic display works perfectly. This is the report from xrandr with the secondary monitor in the system (you'll seem these are just standard HD modes, nothing fancy):

   1280x720      60.00 +  59.94    50.00  
   1920x1080     60.00    59.94    50.00    60.00    50.04

I have tried to follow guidance in other questions on this topic, and tried this sequence of commands:

cvt 1280 720 60
xrandr --newmode "1280x720_60.00"   74.50  1280 1344 1472 1664  720 723 728 748 -hsync +vsync
xrandr --addmode DVI-D-0 1280x720_60.00

After that sequence, I would have expected to use xrandr to set the mode new mode on the output, but the above --addmode command fails. The --addmode attempt is rejected with this error:

$ xrandr --addmode DVI-D-0 1280x720_60.00
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  18 (RRAddOutputMode)
  Serial number of failed request:  47
  Current serial number in output stream:  48

I get a similar complaint if I try a vertical rate of 59.94 too, and if I try to set a 1920x1080 mode. I don't know what to try next. Any suggestions? Did I miss something fundamental? Is there more information I should collect to help understand this?

Thanks for any suggestions!