Thanks for filing the ticket. I’ve replied there.
For posterity, all partitions with exception of ubuntu-boot, ubuntu-save and ubuntu-data are getting created at build time. Those remaining ubuntu-* partitions will be created during install. They should not be placed between existing partitions, this isn’t supported. FWIW I don’t think it’s even possible to support this case reliably without multiple reboots. (i.e. more reboots than what we have now). When ubuntu-data is listed last, it will be resized to fill the remaining space on the disk.