sys-process/cronie ebuild currently has: pkg_setup() { enewgroup crontab } This results in the "crontab" group potentially being assigned a different GID from system to system. I request that sys-process/cronie be migrated to GLEP 81: https://www.gentoo.org/glep/glep-0081.html
Seems to be fixed, as all current testing versions have migrated to GLEP 81.