URL Encoding in Dashboard passing "&" as part of of a value for Query String

I'm working on a summary report in Dashboard version 6.0.5.0, and have URL button that links to detail report in Webi. One of the parameter value we are passing contains "&".

The issue I have is that when we click on the URL link, the passed value gets truncated at the "&" sign in Webi.

As an example, the if value was "Bob&Mary", in the webi prompt, we only see "Bob". URL will show up

/....../BOE/OpenDocument/opendoc/openDocument.jsp?xxx....&lsMpmPrompt1%3A=Bob&Mary...

If I manually change the value from "Bob&Mary" to  "Bob%26Mary”, the  URL encoder changes it to %2226 as per ASCII Encoding Reference (% value is %25).

So my question is that why doesn't the encoder change & to %26 on its own?

Note, I've tested enabling/disabling  "Encode URL" option in the URL button property.

Encode URL For a URL Button, select this checkbox to encode special characters in the URL.

This option ensures that there are no invalid characters.