SNG7 installer doesn't create RAID 1

I'm trying to install SNG7 on a PC with two identical SATA drives and the installer won't create a RAID 1 volume. The PC has a legacy BIOS, it doesn't have UEFI. It will use one disk to create a /boot partition and the other disk to create the Sangoma LVM volume.

I tried 1904 and 1805 and both do the same. I don't know why this is happening, as the installer is supposed to detect that two disks are installed and should automatically create the RAID 1 volume. Instead, it is doing this strange partitioning scheme.

Just to clarify, I'm not doing a customized installation nor using any customized option. I selected the recommended option "FreePBX 14 Installation (Asterisk 13) - Recommended".