The report has a single-value nullable parameter with AvailableValues.
When you preview the report in the Standalone Designer, if you checked Null, the value would be respected.
When you preview the report in Html5 Viewer and the parameter value selection is cleared, instead of rendering the report with a Null value, the report is rendered with the last selected value.

Steps to produce in the Html5 Viewer:

1. Select a value from the filter
2. Click the "Preview" button
3. Report generated based on the selected value
4. Click the "clear selection" button and it clears the selected value from the filter
5. Click the "Preview" button
6. Report is generated based on the value selected before it was cleared rather than with Null value

Here is a link to a muted video on how the issue may be reproduced.

If the parameter is not Nullable, the Preview button after step 4 stays enabled, and clicking on it produces the same result as with the Nullable parameter.