I confirm this bug exists in Lubuntu Next 17.10 amd64, which derives its Ubiquity from Kubuntu.
I can also confirm that the bug did NOT exist in Lubuntu installs 15.10, 16.04, 16.10 all amd64, which is derived from the main Ubuntu Ubiquity.
My workaround was to
1) manually create, format and open the physical volume for encryption
2) launch Ubiquity, and in Manual disk layout choose the /dev/mapper/sdaX_crypt volume as / (root) without formatting
3) complete the installation but choose Continue Testing
4) chroot into target, create crypttab and check fstab, update-initramfs
I confirm this bug exists in Lubuntu Next 17.10 amd64, which derives its Ubiquity from Kubuntu.
I can also confirm that the bug did NOT exist in Lubuntu installs 15.10, 16.04, 16.10 all amd64, which is derived from the main Ubuntu Ubiquity.
My workaround was to sdaX_crypt volume as / (root) without formatting
1) manually create, format and open the physical volume for encryption
2) launch Ubiquity, and in Manual disk layout choose the /dev/mapper/
3) complete the installation but choose Continue Testing
4) chroot into target, create crypttab and check fstab, update-initramfs
for step-by-step instructions see https:/ /github. com/artmg/ lubuild/ blob/master/ help/configure/ LxQt-Kubuntu- Ubiqity- manual- encryption- bug.md