Ubuntu 16.04 LTS (Xenial Xerus) released

Yeah, some initramfs scripts can do some funky things. When I code an initramfs, my philosophy is, if you mounted /proc or /sys because you had to for some reason, you unmount them before switch_root. But not everyone follows that, and it works because everyone kind-of expects /proc and /sys and devtmpfs to sometimes be mounted by random crap -- or, more accurately, no one notices when they are, because they get mounted very quickly anyway. That's sloppy, though, and it's best if we don't add more pseudo-fses to the slop.

Also, btw, initrd has been deprecated for about a decade.