Failed to lookup gem object for mapping

Hello, I have a laptop with Intel integrated graphics paired with an NVidia GeForce 950M card.
Since the HDMI port is connected to the NVidia chip, in order to use an external monitor until now I configured Xorg to use NVidia only. However I was wondering if there was a way to use Intel by default and NVidia for certain applications while still retaining the external monitor.

I followed this guide for reverse PRIME on the Arch Wiki (I also had to do this for it to work). Now the output of xrandr --listproviders is:

Providers: number : 2
Provider 0: id: 0x44 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 3 outputs: 2 associated providers: 1 name:modesetting
Provider 1: id: 0xcc cap: 0x2, Sink Output crtcs: 4 outputs: 1 associated providers: 1 name:modesetting

The external monitor is detected; the output of xrandr -q is:

Screen 0: minimum 320 x 200, current 3839 x 1080, maximum 16384 x 16384
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
   1920x1080     60.01*+  60.01    59.97    59.96    59.93    40.00  
   1680x1050     59.95    59.88  
   1400x1050     59.98  
   1600x900      59.99    59.94    59.95    59.82  
   1280x1024     60.02  
   1400x900      59.96    59.88  
   1280x960      60.00  
   1440x810      60.00    59.97  
   1368x768      59.88    59.85  
   1280x800      59.99    59.97    59.81    59.91  
   1280x720      60.00    59.99    59.86    59.74  
   1024x768      60.04    60.00  
   960x720       60.00  
   928x696       60.05  
   896x672       60.01  
   1024x576      59.95    59.96    59.90    59.82  
   960x600       59.93    60.00  
   960x540       59.96    59.99    59.63    59.82  
   800x600       60.00    60.32    56.25  
   840x525       60.01    59.88  
   864x486       59.92    59.57  
   700x525       59.98  
   800x450       59.95    59.82  
   640x512       60.02  
   700x450       59.96    59.88  
   640x480       60.00    59.94  
   720x405       59.51    58.99  
   684x384       59.88    59.85  
   640x400       59.88    59.98  
   640x360       59.86    59.83    59.84    59.32  
   512x384       60.00  
   512x288       60.00    59.92  
   480x270       59.63    59.82  
   400x300       60.32    56.34  
   432x243       59.92    59.57  
   320x240       60.05  
   360x202       59.51    59.13  
   320x180       59.84    59.32  
HDMI-1 disconnected (normal left inverted right x axis y axis)
HDMI-1-2 connected (normal left inverted right x axis y axis)
   1920x1080     60.00 +  74.97    50.00    59.94  
   1680x1050     59.95  
   1280x1024     75.02    60.02  
   1440x900      59.89  
   1280x960      60.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    70.07    60.00  
   800x600       72.19    75.00    60.32    56.25  
   720x576       50.00  
   720x480       59.94  
   640x480       75.00    72.81    59.94    59.93  
  1680x1050 (0x4b) 146.250MHz -HSync +VSync
        h: width  1680 start 1784 end 1960 total 2240 skew    0 clock  65.29KHz
        v: height 1050 start 1053 end 1059 total 1089           clock  59.95Hz
  1280x1024 (0x52) 108.000MHz +HSync +VSync
        h: width  1280 start 1328 end 1440 total 1688 skew    0 clock  63.98KHz
        v: height 1024 start 1025 end 1028 total 1066           clock  60.02Hz
  1280x960 (0x55) 108.000MHz +HSync +VSync
        h: width  1280 start 1376 end 1488 total 1800 skew    0 clock  60.00KHz
        v: height  960 start  961 end  964 total 1000           clock  60.00Hz
  1024x768 (0x63) 65.000MHz -HSync -VSync
        h: width  1024 start 1048 end 1184 total 1344 skew    0 clock  48.36KHz
        v: height  768 start  771 end  777 total  806           clock  60.00Hz
  800x600 (0x72) 40.000MHz +HSync +VSync
        h: width   800 start  840 end  968 total 1056 skew    0 clock  37.88KHz
        v: height  600 start  601 end  605 total  628           clock  60.32Hz
  800x600 (0x73) 36.000MHz +HSync +VSync
        h: width   800 start  824 end  896 total 1024 skew    0 clock  35.16KHz
        v: height  600 start  601 end  603 total  625           clock  56.25Hz
  640x480 (0x7f) 25.175MHz -HSync -VSync
        h: width   640 start  656 end  752 total  800 skew    0 clock  31.47KHz
        v: height  480 start  490 end  492 total  525           clock  59.94Hz

However, when I try to enable it with xrandr --output HDMI-1-2 --auto, the output I get is:

xrandr: Configure crtc 3 failed

And I also get the following messages in dmesg:

[ 3823.706500] [drm:nv_drm_dumb_map_offset [nvidia_drm]] *ERROR* [nvidia-drm] [GPU ID 0x00000100] Failed to lookup gem object for mapping: 0x00000006
[ 3823.706533] [drm:nv_drm_dumb_map_offset [nvidia_drm]] *ERROR* [nvidia-drm] [GPU ID 0x00000100] Failed to lookup gem object for mapping: 0x00000007

When I do it with a graphical program (KDE system settings) nothing happens.
My distribution is Arch Linux, NVidia is 470.63.01.
Doing this on Wayland seems to work, although the external screen is pretty laggy.

EDIT: my situation is pretty much identical to the one described here: video - Mysterious problem on setting up Reverse Prime (NVIDIA Optimus offloading through OutputSink) for an HDMI external monitor - Unix & Linux Stack Exchange