Yes, the module works only for single select fields. That fact is stated on this page, see the phrase above "Note that this mod affects only select Selector (Select (Drop-Down Box)) controls.". I.e. it can't work for Multiple-Select type fields. The search form works because it is treated as a special case and it can produce unpredictable behavior if there would be a triple dependency, that is why there is an instruction on how to turn that feature off.