Comment 2 for bug 1719176

I can confirm this problem still persists for Ubuntu 20.04 LTS (haven't tested on anything more recent). Here's the crypttab(5) I used for testing:

crypt-tmp /dev/disk/by-partlabel/linux-tmp /dev/urandom cipher=aes-xts-plain64,size=256,hash=sha1,tmp=ext4

This is a fairly serious security bug because it will fail quietly. The only way to tell is to see the journal message or check manually whether /tmp is mounted from a LUKS volume. If anybody attempts to set up an encrypted /tmp this way without checking it actually gets mounted, they'll get a /tmp that's just a plain old directory in the root file system.