Upon updating to the new 2.02_beta3 release from the previous unstable release my system suddenly became unbootable. Looking at the grub.cfg Grub is attempting to use root=UUID to boot due to my intel microcode-only initrd. I had previously set GRUB_DISABLE_LINUX_UUID to true in /etc/default/grub in order to prevent that from happening (instead it would generate root=/dev/sdX), which worked fine until this update.
Fixed in grub-2.02_beta3-r1. Thanks for the report! https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=20850e98766f137b01b6df134ed90bef9bc3d5e0