Comment 22 for bug 1510731

I found a somewhat simpler workaround for this (17.04 and 17.10):

On the "Installation type" screen, I select guided partitioning with luks & lvm. This way the two fields for the passwords appear. Then, I enter my desired password, switch back to manual partitioning, and click next.

This way, when I create the physical volume for encryption, the encrypted volume shows up as expected.