Using two GPUs with one monitor each, one is always black

Using two GPUs with one monitor each, one is always black. Although the cursor does move between them and I can see it on the black monitor too.

Mutter seems to log about both of them with a valid layout:

VERBOSE: Binding monitor 0x564856d74280/EP-HDMI-RX (1920, 0, 1280, 720) x 60.000500
VERBOSE: Binding monitor 0x564856d742d0/DELL U2413 (0, 0, 1920, 1200) x 59.950672
WORKAREA: Computed work area for workspace 0 monitor 0: 1920,0 1280 x 720
WORKAREA: Computed work area for workspace 0 monitor 1: 0,0 1920 x 1200
WORKAREA: Computed work area for workspace 0 monitor 0: 1920,0 1280 x 720
WORKAREA: Computed work area for workspace 0 monitor 1: 0,27 1920 x 1173

Only the DELL monitor (Intel GPU) is black. The EP-HDMI-RX (Nouveau) is painted correctly.

Annoyingly, the DELL is also my primary monitor so I don't get to see the shell at all.

Trying mir_demo_server for comparison it seemed to have similar trouble at first, so maybe it's a kernel issue. After a while it was able to paint but I was not able to get that screen to change at all. Possibly for unrelated reasons.