I just posted at https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1606103 that the problem seems to be in hw/xfree86/drivers/modesetting/drmmode_display.c#add_gtf_modes(), which is forcing a maximum vertical refresh of 60 (plus 1%), even though my monitor can actually do up to 120, and so any modelines with a refresh above this are being flagged as out of range and being pruned later.
It looks like the function that calls add_gtf_modes() has access to the monitor info, including vertical refresh, so a solution might be to pass the info to add_gtf_modes() and have add_gtf_modes() use this maximum refresh. That's assuming that add_gtf_modes() is being called once per monitor rather than for all monitors.
