The attached screenshot is the result of running the preview of the demo project (also in the attachments). The default HTML5 Viewer's editor for multivalue report parameters is a Kendo UI List, where you can clear the selection or select all values with the options beneath the editor.
If I am misunderstanding the issue or I am missing a setting, please post details what should be updated in the attached demo project in order to reproduce the problem. Let us know also the tested browsers.
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