After looking at those logs I thought the problem was too small of a disk. It looked like partman realized it could not resize the existing volumes to make enough room for a new install, then fell back to erasing the whole disk. Unfortunately, I am not able to reproduce it, even using a disk and starting partitions exactly the same size as in the logs. It simply does not give the option to do a side by side install. Are you sure you chose that option?
After looking at those logs I thought the problem was too small of a disk. It looked like partman realized it could not resize the existing volumes to make enough room for a new install, then fell back to erasing the whole disk. Unfortunately, I am not able to reproduce it, even using a disk and starting partitions exactly the same size as in the logs. It simply does not give the option to do a side by side install. Are you sure you chose that option?