I think the root cause here is the same as mentioned in Sysupgrade 22.03 to 22.03.1 on x86 fails with unable to determine upgrade device - #17 by D43m0n -- the resizing process changes the root partition UUID, which then confuses the upgrade logic sysupgrade / auc, Attended sysupgrade appears to rely on to map the root partition to root device.
fdisk does appear to let you set the UUID in expert mode, so maybe the disk resize instructions should follow-up with a reinstating of the former partition UUID once it's been recreated to fill the remainder of the physical disk, rather than recommending fixing up the fallout of that change (e.g. updating GRUB with new root partition UUID), since the UUID change causes other downstream issues?