I just tested it and the problem is that the dm_cache_smq module is missing from initramfs. Adding it to "/etc/initramfs-tools/modules" and running "update-initramfs -u" addresses the problem.
I guess that lvm2 should add dm_cache and dm_cache_smq to /usr/share/initramfs-tools/hooks/lvm2, just like it has dm_mirror and dm_raid.
