Selection: dynamic multi-select list does not save selected values

The selected values in a dynamic multi-select list are not saved.

Steps to reproduce:
1. login to the CMS of the selection plugin demo project
2. in content/en, create a new document of type 'textdocument'
3. select some values in the field 'DynamicMultiSelect - List'
4. save & close the document

Expected: all selected values in the 'DynamicMultiSelect - List' field are saved
Actual: none of the selected values in the 'DynamicMultiSelect - List' field is saved

N.B. the underlying cause may be that the 'selection:multiselectlist' property of the new document variant is of type Double, which should be a String instead.