FWIW: I was trying to reproduce this issue last week without success. My pi3-1 was flashed with the edge image from http://cdimage.ubuntu.com/ubuntu-core/16/edge/current/ubuntu-core-16-armhf+raspi3.img.xz
My SD card details:
zyga@localhost:/sys/class/block/mmcblk0/device$ grep "" * 2>/dev/null
cid:02544d534130384706249800bd00b6d3
csd:400e00325b5900003b4f7f800a40004b
date:06/2011
erase_size:512
fwrev:0x6
hwrev:0x0
manfid:0x000002
name:SA08G
oemid:0x544d
preferred_erase_size:4194304
scr:0235800001000000
serial:0x249800bd
type:SD
uevent:DRIVER=mmcblk
uevent:MMC_TYPE=SD
uevent:MMC_NAME=SA08G
uevent:MODALIAS=mmc:block
I ran the following test:
SPREAD_EXTERNAL_ADDRESS=192.168.3.3:22 spread -v -debug -reuse -repeat 5 external:ubuntu-core-16-arm-32:tests/main/ubuntu-core-upgrade