Same issue with my Ultrawide. Workaround: xrandr --newmode "2560x999_60.00" 211.75 2560 2720 2984 3408 999 1002 1012 1037 -hsync +vsync xrandr --addmode Virtual1 2560x999_60.00 xrandr --output Virtual1 --mode 2560x999_60.00
Same issue with my Ultrawide.
Workaround:
xrandr --newmode "2560x999_60.00" 211.75 2560 2720 2984 3408 999 1002 1012 1037 -hsync +vsync
xrandr --addmode Virtual1 2560x999_60.00
xrandr --output Virtual1 --mode 2560x999_60.00