After more debugging, it turned out that the regression was in fact the effect
of fixing LP#1561643: initrmafs-tools kept installing fb modules even when
framebuffer support was not enabled, and kernel snap never explicitly enabled
framebuffer initramfs-tools script, but still came bundled with such kmods.
In Ubuntu Classic, to bundle fb kmods in initrd, we normally install cryptsetup:
so to avoid any possible regression in kernel snaps and to keep the changes to a
minimum, we avoid installing cryptsetup and just enable framebuffer in
initramfs-tools/conf-hooks.d (see the attached path).
How to test:
A diff between the content of an 'empty initrd' and one that is generated after
applying the above fix, shows that fb kmods are now back: