try the below code, I have added Mult="TRUE" this we used to use in our old codes
var fieldSchema = <Field Type="TaxonomyFieldTypeMulti"
If you execute the items API call without any filter, you will get a result set that contains all the items in that list. Maybe not a problem if the list only contains a small number of items, but obviously lists can potentially contain thousands of items and that would not perform well. The following example gets just retrieves the most recently modified ...