Comment 11 for bug 1506139

More info from the source code of systemd-cryptsetup:

 else if (streq(option, "luks"))
                arg_type = CRYPT_LUKS1;
...
 } else if (STR_IN_SET(option, "plain", "swap", "tmp"))
                arg_type = CRYPT_PLAIN;

so the swap argument overwrites the luks argument and resets the ecryption type to plain.

And indeed, when using

/lib/systemd/systemd-cryptsetup attach sda2_crypt /dev/sda2 none swap,luks,discard

(i.e. just change the order of the parameters, use swap,luks,discard instead of luks, swap, discard , as the ubuntu installer creates, it works and uses the luks partition correctly.