Thank you npadmani for the reply.
The vRO action is returning a type of: Array/string.
The value of the property on the blueprint is string, and I have specified a default value of "Invalid Default Value":
This default value never shows in the catalog, as the vRO action runs immediately when the catalog request is opened. On a fresh catalog request, before the feeder property 'Environment' is selected, the action returns this (Environment is undefined, so the parent OU is undefined):
Once an Environment is selected, the vRO action returns a proper value:
The value of the OU field changes appropriately if/when the Environment property selection is changed:
The vRO action appears to be working properly. Everything looks good until I hit submit, and then vRA complains about the value of this field.