I installed memtest86, but for some reason, running grub-mkconfig as directed failed to install a menu item for memtest86.
Created attachment 468166 [details] emerge --info
Ok so it DOES in fact manage to show up on the grub menu after I reboot. I didn't, however, see a message about it pop up during grub-mkconfig.
This is not a bug in sys-boot/grub. I believe this was fixed by https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=b2f095cc8a812ca6b9e57dafc2825b7041c41aec.