The change in udev behavior as described in this article https://www.gentoo.org/support/news-items/2016-11-04-important_fstab_and_localmount_update.html can also cause wpa_supplicant to malfunction. If wpa_supplicant starts before the network device is settled and the proper kernel module is not loaded yet then wpa_supplicant startup fails. This happens with my fast new laptop with nvme storage. Adding rc_want="dev-settle" to /etc/conf.d/wpa_supplicant helps to work around this. My suggestion would be to do this by default, or add this to /etc/init.d/wpa_supplicant.
Since this isn't going to be needed everywhere, I would tend to recommend putting it in /etc/conf.d/wpa_supplicant if you need it rather than putting it in the init script by default.
added as a comment to confd file. users will get the change on update/rebuild but I am not bumping to add a comment.
