This isn't related to grub. The LUKS password prompt is on one of the text ttys ( switch ttys to find it ) and waiting for you to unlock the volume. The prompt is on a tty because plymouth is not installed into the initramfs because it has no themes installed. It seems that lubuntu-core depends on a theme and so should pull it in, but lubuntu-core is not being installed either.
This isn't related to grub. The LUKS password prompt is on one of the text ttys ( switch ttys to find it ) and waiting for you to unlock the volume. The prompt is on a tty because plymouth is not installed into the initramfs because it has no themes installed. It seems that lubuntu-core depends on a theme and so should pull it in, but lubuntu-core is not being installed either.