In a recent client solution we were utilizing a multi-valued Managed Metadata field to classify documents in a organizational taxonomy. Our search and navigation relied on being able to retrieve documents by supplying any one of the possibly multiple values stored in the Managed Metadata field.
We determined however, that even though many documents had multiple values in this field, they were returning only one value in the managed property.
We examined the crawled property feeding the managed property and found IsMultiValued = false, and figured that was the issue. It turns out, this property is not used in FAST.
Configuring multi-value properties
If you want to use query refinement, don’t forget to set the property RefinementEnabled and enable the query refiner in the front-end (reference to come).
After setting the Merge Crawled Properties property on the managed property we were able to get multiple values back for the Managed Metadata field.