I found a small typo in the make.conf(5) man page (portage-2.1.9.43). PORT_LOGDIR This variable defines the directory in which per-ebuild logs are kept. Logs are created only when this is set. They are stored as ${CATEGORY}:${PF}:YYYYMMDD-HHMMSS.log in the directory speci- fied. If the direcory does not exist, it will be created auto- matically and group permissions will be applied to it. If the directory already exists, portage will not modify it's permis- sions. Notice the "direcory" instead of "directory".
Thanks, this is fixed in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=8ab4771b6392e5e552e2a159d79c9b7eb7ef3d63
This is fixed in 2.1.9.44 and 2.2.0_alpha28.