Hi Endprodukt,
You can try using different sync polarities for the 15 and 31 kHz ranges:
vmmaker.ini
MonitorType = "CUSTOM"
monitor_specs0 15700-15800, 45-80, 4.000, 4.700, 5.300, 0.062, 0.062, 0.740, 0, 0, 288, 448
monitor_specs1 16100-16200, 45-80, 1.280, 4.500, 6.420, 0.062, 0.062, 0.740, 0, 0, 288, 448
monitor_specs2 25100-25200, 45-80, 0.200, 0.400, 8.000, 0.040, 0.040, 0.640, 0, 0, 384, 768
monitor_specs3 31500-31600, 45-80, 0.170, 0.350, 5.500, 0.040, 0.040, 0.640,
1, 1, 576, 800
mame.ini
monitor custom
crt_range0 15700-15800, 50-65, 1.800, 0.400, 7.400, 0.064, 0.160, 1.056, 0, 0, 192, 256, 0, 0
crt_range1 16200-16300, 50-65, 0.200, 0.400, 8.000, 0.040, 0.040, 0.640, 0, 0, 256, 264, 512, 528
crt_range2 25300-25400, 50-65, 0.200, 0.400, 8.000, 0.040, 0.040, 0.640, 0, 0, 384, 400, 768, 800
crt_range3 31500-31600, 50-65, 0.170, 0.350, 5.500, 0.040, 0.040, 0.640,
1, 1, 400, 512, 0, 0
This may or may not work

On a side note, I'd say the best option for this monitor in the long term is to always use the 31 kHz range with software scanlines (in mame.ini, "effect scanlines"), using ultrawide resolutions with "cleanstretch 2". The 31 kHz range has the best geometry with this particular monitor. In theory, the picture should look exactly the same as with true 15 kHz modes. Unfortunately, in my experience, brightness gets greatly reduced when doing this in MAME. I'm not sure if it has to do with the way the filter is applied or is monitor related.