If the parent page displays in IE11 document mode, this mode will also be inherited by the iframe in the dialog (modal) pop-up, such that the SSRS report viewer also renders in IE11 document mode — instead of the document mode defined for SSRS reports if rendered directly. In my environment, this is IE5 to ensure that the report parameters and toolbar buttons function correctly in IE11, which most of users still use as their primary browser.
Prior to displaying the impacted SSRS report in a dialog pop-up, I had configured my site to launch it in a separate tab/window from the report server (using the report server URL access approach described here). For the embedded report links that I wanted to open in a new tab/window, I set the link’s action (technically, the action of the tablix cell or placeholder, depending on your design) to “Go to URL” and used the typically-recommended expression window.open(url, ‘_blank’).
Resolution / Workaround