First thought I'd suggest:
Look at the cable for the monochrome monitor and see if pin 9 is present. On the original VGA that was an unused "key" line, but on cards supporting VESA DDC2 that line is a +5v power line. It might be possible that the mono monitor has pin 9 wired to ground, which would short that power supply.
Second thought:
The sense line on Pin 12 should be grounded in a fixed-frequency VGA monitor. Pin 12 was reused as the i2c serial bus line on VESA DDC. (which means that DDC and monochrome are exclusionary.) Perhaps the Mac's video card freaks out at having that line pulled to ground while the BIOSes on the PC cards still acknowledge the pre-DDC usage of the sense lines and adjust accordingly.
Look at the cable for the monochrome monitor and see if pin 9 is present. On the original VGA that was an unused "key" line, but on cards supporting VESA DDC2 that line is a +5v power line. It might be possible that the mono monitor has pin 9 wired to ground, which would short that power supply.
Second thought:
The sense line on Pin 12 should be grounded in a fixed-frequency VGA monitor. Pin 12 was reused as the i2c serial bus line on VESA DDC. (which means that DDC and monochrome are exclusionary.) Perhaps the Mac's video card freaks out at having that line pulled to ground while the BIOSes on the PC cards still acknowledge the pre-DDC usage of the sense lines and adjust accordingly.