Here's some more info! There is a conflict between two settings under "Control key position". When I first opened the dialog, the "At bottom left" option was already selected. If that option is selected, the option "Make Caps Lock an additional Ctrl" has no effect. If, on the other hand, I deselect "At bottom left", "Make Caps Lock an additional Ctrl" works as desired.
> Control key position
[ ] At bottom left
...
[ ] Make Caps lock an additional Ctrl
==>
lock Caps_Lock (0x42)
control Control_L (0x25), Control_R (0x69)
Here's some more info! There is a conflict between two settings under "Control key position". When I first opened the dialog, the "At bottom left" option was already selected. If that option is selected, the option "Make Caps Lock an additional Ctrl" has no effect. If, on the other hand, I deselect "At bottom left", "Make Caps Lock an additional Ctrl" works as desired.
> Control key position
[ ] At bottom left
...
[ ] Make Caps lock an additional Ctrl
==>
lock Caps_Lock (0x42)
control Control_L (0x25), Control_R (0x69)
******* ******* ******* ******* ******* ******* ******* *******
> Control key position
[ ] At bottom left
...
[X] Make Caps lock an additional Ctrl
==>
lock
control Control_L (0x25), Control_L (0x42), Control_R (0x69)
******* ******* ******* ******* ******* ******* ******* *******
> Control key position
[X] At bottom left
...
[X] Make Caps lock an additional Ctrl
==>
lock Caps_Lock (0x42)
control Control_L (0x25), Caps_Lock (0x42), Control_R (0x69)