nouveau messages with 2 GPUs

Using 2 GPUs on a desktop (nouveau+amdgpu, or nouveau+nouveau), each one connected to a single display.

kernel 4.18.9

If /dev/dri/card0 is an nvidia card:

03:00.0 VGA compatible controller: NVIDIA Corporation GM107GL [Quadro K620] (rev a2)
0b:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Baffin [Radeon RX 550 640SP / RX 560/560X] (rev cf)

dmesg keeps getting flooded with these messages:

Oct 25 15:10:41 linux-nq5u kernel: nouveau 0000:03:00.0: gr: DATA_ERROR 0000009c [] ch 15 [007ed37000 systemd-logind[888]] subc 0 class b097 mthd 17e0 data 0000000c
Oct 25 15:10:41 linux-nq5u kernel: nouveau 0000:03:00.0: gr: DATA_ERROR 0000009c [] ch 15 [007ed37000 systemd-logind[888]] subc 0 class b097 mthd 0d78 data 00000004
Oct 25 15:10:41 linux-nq5u kernel: nouveau 0000:03:00.0: gr: DATA_ERROR 0000009c [] ch 15 [007ed37000 systemd-logind[888]] subc 0 class b097 mthd 0d78 data 00000004
Oct 25 15:10:41 linux-nq5u kernel: nouveau 0000:03:00.0: gr: DATA_ERROR 0000009c [] ch 15 [007ed37000 systemd-logind[888]] subc 0 class b097 mthd 0d78 data 00000004
Oct 25 15:10:41 linux-nq5u kernel: nouveau 0000:03:00.0: gr: DATA_ERROR 0000009c [] ch 15 [007ed37000 systemd-logind[888]] subc 0 class b097 mthd 0d78 data 00000004

Looks like systemd-logind keeps trying to interact with the card. Asa far as I see systemd-logind only calls DRM_IOCTL_SET_MASTER and DRM_IOCTL_DROP_MASTER ioctl on the card.
Also, wayland/mutter only displays on one display, the other one stays blank.

The problem also appears with 2 nvidia cards (no amd).

If /dev/dri/card0 is not an nvidia card, i.e. we switch pci slots:

03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Baffin [Radeon RX 550 640SP / RX 560/560X] (rev cf)
0b:00.0 VGA compatible controller: NVIDIA Corporation GM107GL [Quadro K620] (rev a2)

there is no issue. No dmesg issues, and wayland/mutter works fine on both displays.

In all cases, there are also these nouveau messages at boot-time:

Oct 25 10:14:26 linux-nq5u kernel: nouveau 0000:03:00.0: bus: MMIO read of 00000000 FAULT at 3e6684 [ IBUS ]
Oct 25 10:14:26 linux-nq5u kernel: nouveau 0000:03:00.0: bus: MMIO read of 00000000 FAULT at 10ac08 [ IBUS ]