Comment # 1 on bug 960463 from Neil Rickert

I always install that way.

I ignore the proposed partitioning, and instead click on "Create partitioning".

On the next screen, I select "Custom partitioner".

That lists the existing partitions and LVM volumes.  I then right-click on
those and choose "Edit" to change how they will be used.

This worked fine for me with 42.1 installs.