I'm unsure if I should file a new bug at this point, but here is an update for
the LTS kernel:

4.19.20 breaks functionality again, now the second screen is reliably dead at
1080p (native resolution). I could bisect down to
0857b8439b6b9ce388e47652420ac083ea5f08d6 by Yogesh Mohan Marimuthu -
drm/amd/display: calculate stream->phy_pix_clk before clock mapping.

It's the same for 4.20.17 and 5.0.6, I assume they contain the patch, too. I
will attach debug kernel logs for both 4.19.20+ before the patch (working after
screen reset as described above) and after.