sysloggood.log = 20180131
syslogbad.log = 20180214

I stripped the date prefix from the two logs to simplify comparison. 
e.g. cat syslog | sed "s/Feb 14 ..:..:..//" > sysloggood.log

I noticed in a few places it has debconf entries for .. "load-cdrom, load-iso" .. but in the bad logs the "load-cdrom" part is missing.  

This may be relevant as the cdrom source is "missing" later in the install:

e.g. GOOD:
 debconf: --> SUBST anna/choose_modules CHOICES-C choose-mirror, download-installer, fdisk-udeb, iso-scan, load-cdrom, load-iso, load-media, lowmem, mouse-modules-4.13.0-25-generic-di, multipath-modules-4.13.0-25-generic-di, network-console, oem-config-udeb, openssh-client-udeb, parted-udeb, reiserfsprogs-udeb, rescue-mode, vlan-modules-4.13.0-25-generic-di
 debconf: Adding [CHOICES-C] -> [choose-mirror, download-installer, fdisk-udeb, iso-scan, load-cdrom, load-iso, load-media, lowmem, mouse-modules-4.13.0-25-generic-di, multipath-modules-4.13.0-25-generic-di, network-console, oem-config-udeb, openssh-client-udeb, parted-udeb, reiserfsprogs-udeb, rescue-mode, vlan-modules-4.13.0-25-generic-di]

e.g. BAD:
 debconf: --> SUBST anna/choose_modules CHOICES-C choose-mirror, download-installer, fdisk-udeb, iso-scan, load-iso, load-media, lowmem, mouse-modules-4.13.0-32-generic-di, multipath-modules-4.13.0-32-generic-di, network-console, oem-config-udeb, openssh-client-udeb, parted-udeb, reiserfsprogs-udeb, rescue-mode, vlan-modules-4.13.0-32-generic-di
 debconf: Adding [CHOICES-C] -> [choose-mirror, download-installer, fdisk-udeb, iso-scan, load-iso, load-media, lowmem, mouse-modules-4.13.0-32-generic-di, multipath-modules-4.13.0-32-generic-di, network-console, oem-config-udeb, openssh-client-udeb, parted-udeb, reiserfsprogs-udeb, rescue-mode, vlan-modules-4.13.0-32-generic-di]

note: there is no "load-cdrom" option in the bad one...


---

Good:
 debconf-copydb: Adding [CHOICES-C] -> [choose-mirror, download-installer, fdisk-udeb, iso-scan, load-cdrom, load-iso, load-media, lowmem, mouse-modules-4.13.0-25-generic-di, multipath-modules-4.13.0-25-generic-di, network-console, oem-config-udeb, openssh-client-udeb, parted-udeb, reiserfsprogs-udeb, rescue-mode, vlan-modules-4.13.0-25-generic-di]
 debconf-copydb: Adding [CHOICES] -> [choose-mirror: Choose mirror to install from (menu item), download-installer: Download installer components, fdisk-udeb: Manually partition a hard drive (fdisk), iso-scan: Scan hard drives for an installer ISO image, load-cdrom: Load installer components from CD, load-iso: Load installer components from an installer ISO, load-media: Load installer components from removable media, lowmem: free memory for lowmem install, mouse-modules-4.13.0-25-generic-di: Mouse support, multipath-modules-4.13.0-25-generic-di: DM-Multipath support, network-console: Continue installation remotely using SSH, oem-config-udeb: Prepare for OEM configuration, openssh-client-udeb: secure shell client for the Debian installer, parted-udeb: Manually partition a hard drive (parted), reiserfsprogs-udeb: User-level tools for ReiserFS filesystems, rescue-mode: mount requested partition and start a rescue shell, vlan-modules-4.13.0-25-generic-di: vlan modules]

bad:
 debconf-copydb: Adding [CHOICES-C] -> [choose-mirror, download-installer, fdisk-udeb, iso-scan, load-iso, load-media, lowmem, mouse-modules-4.13.0-32-generic-di, multipath-modules-4.13.0-32-generic-di, network-console, oem-config-udeb, openssh-client-udeb, parted-udeb, reiserfsprogs-udeb, rescue-mode, vlan-modules-4.13.0-32-generic-di]
 debconf-copydb: Adding [CHOICES] -> [choose-mirror: Choose mirror to install from (menu item), download-installer: Download installer components, fdisk-udeb: Manually partition a hard drive (fdisk), iso-scan: Scan hard drives for an installer ISO image, load-iso: Load installer components from an installer ISO, load-media: Load installer components from removable media, lowmem: free memory for lowmem install, mouse-modules-4.13.0-32-generic-di: Mouse support, multipath-modules-4.13.0-32-generic-di: DM-Multipath support, network-console: Continue installation remotely using SSH, oem-config-udeb: Prepare for OEM configuration, openssh-client-udeb: secure shell client for the Debian installer, parted-udeb: Manually partition a hard drive (parted), reiserfsprogs-udeb: User-level tools for ReiserFS filesystems, rescue-mode: mount requested partition and start a rescue shell, vlan-modules-4.13.0-32-generic-di: vlan modules]

Where are these d-i/debconf values being set?