It looks like you're new here. If you want to get involved, click one of these buttons!
Cache Mobs have very tiny souls.
Could it be the spawn that DON'T have "Guardian" tags normally because they are tamable (cold drake, Frost Dragon, Dragon Wolf), and their Ancient Chest Guardian counterparts are the ones with issues?
In all seriousness, it just took 1 Stash+ 1 Supply to get the first step, and 3 more supply to get to Grand, on a soulbinder.