Hi!
We’ve got a case where the system failed to boot because an extra partition was inserted between ubuntu-save and the ubuntu-data partitions. This is not supported at the moment as explained in the forum thread, so the solution is to place any extra partitions before the ubuntu-boot one.
This is probably something that should be mentioned in the documentation: in the volumes/structures map, ubuntu-data needs to be the last partition and no extra partitions can be inserted between the sequence ubuntu-boot - ubuntu-save - ubuntu-data: if extra partitions are required, they need to be declared before ubuntu-boot.