Current stable version of genkernel (188.8.131.52) builds initramfs images without proper support for LVM cache (ssd as hdd cache). Thus LVM cached root/partitions are unavailable at boot time.
To boot properly sys-block/thin-provisioning-tools utils must be included + its c++ dependencies (libexpat.so*, libgcc_s.so* and libstdc++.so*)
I suggest a separate command-line argument for genkernel to include those.
For details see https://forums.gentoo.org/viewtopic-t-1014256.html