If enewuser is called in the context of pkg_*, it ignores ${ROOT} and creates the user on the local system. This isn't an easy problem to fix, IMHO. The user needs to be added to ${ROOT}/etc/passwd, but it also needs to be known to the local system so that file ownership can be set correctly when chown is called from the ebuild. It seems like there's no choice but to (1) find a unique user id that is the same for /etc/passwd and ${ROOT}/etc/passwd, (2) create the user in both places.
*** This bug has been marked as a duplicate of 42320 ***