I think comment #75 is onto something. d-i/source/partman-efi/fstab.d/efi first looks for a partman-auto/disk question and will use the device it specifies to look for an efi partition, otherwise it uses the first one it can find from any device. I can't find anywhere where ubiquity is setting that question, so a quick fix may be to have ubiquity set it. It should be given the same value as the grub-installer/bootdev question.
I think comment #75 is onto something. d-i/source/ partman- efi/fstab. d/efi first looks for a partman-auto/disk question and will use the device it specifies to look for an efi partition, otherwise it uses the first one it can find from any device. I can't find anywhere where ubiquity is setting that question, so a quick fix may be to have ubiquity set it. It should be given the same value as the grub-installer/ bootdev question.