Original Summary:
vtty[1-6] nouveau framebuffers produce vast snow/raster on right and bottom border areas on the larger resolution of two displays, in the absence of both plymouth and video= on linu lines

FTR: Upstream bugs:
https://bugzilla.kernel.org/show_bug.cgi?id=217479 (includes most detail, w/ dmesg)
https://gitlab.freedesktop.org/drm/nouveau/-/issues/214

To reproduce:
1-connect two unequal native resolution displays to a Tesla or Firmi GPU
2-don't have plymouth in use (I don't ever have it installed, so don't know whether it impacts)
3-don't include e.g. video=1440x900@60 directive on Grub's linu lines
4-boot TW
5-switch to a vtty, e.g. Ctrl-Alt-F3

Actual behavior:
1-Both displays utilize the resolution (same pixel grid) of the lower resolution display
2-Lower resolution display behaves as expected (light text on black background)
3-Higher resolution display uses same pixels as lower resolution display, with light text on black background, leaving right side and bottom raster instead of black

Expected behavior:
1-Both displays utilize the resolution (same pixel grid) of the lower resolution display
2-Lower resolution display behaves as expected
3-Entire higher resolution display's background is black instead of portions in raster

Workaround: add e.g. video=1440x900@60 to Grub's linu lines, which causes both displays to use the same nominal mode on the full display space.

Occurs with TW 6.3.2, 6.2.12 & 6.2.4 kernel-default, and with 6.2.15 on Fedora 38 and Mageia 9.
Does not occur with TW 6.1.12 kernel-default on NVidia, or with AMD Caicos (Terascale2) GPU, or with Intel Eaglelake GPU.
Tested only on legacy booting (no UEFI support).
Others might describe what I call "raster" as multicolored snow.
nosimplefb=1 doesn't change anything.