By default, /etc/crypttab is not present in initramfs because the default unlock script does not use that location; it uses /cryptroot/crypttab instead.
To make /etc/crypttab available as /cryptroot/crypttab in initramfs, create the following script in the /etc/initramfs-tools/hooks directory and make it executable:
#!/bin/sh
cp /etc/crypttab "${DESTDIR}/cryptroot/crypttab"
exit 0