Minor Description
The Collapse QParser does give results if you collapse on a multi-valued field, but the document->value is somewhat arbitrarily chosen based on the internals of the FieldCache (FieldCacheImpl.SortedDocValuesCache).
Note that the Grouping functionality accesses values via FieldType.getValueSource which is a layer of abstraction above that includes a multiValued error check.
Collapse should throw an error here.
p.s. easy to test with exampledocs collapsing on "cat"