This is probably a bug that is not reported, the spirits are not going past enemy defenses as though they are walls and the A.I. are targeting them for destruction.
Thanks for letting us know. We'll add this to our backlog to look into, but it is likely to be extremely low priority and I cannot confirm the chances of a fix at this time.
I am aware that it would be on the low end since a lot of people probably don't use spirits as scouts.
I was able to rally them past a rampart and possess one over a rampart.
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