Update: The October 2015 Cumulative Update package resolves this issue.
MS15-036 (KB2965219) and the April 2015 CU include the following change:
Assume that you type a page name, such as “Text with spaces,” in the New item form in a site page library in SharePoint Server 2013 to create a new page. After you create the page, the automatically generated URL is inconsistent with the preview URL. For example, the preview URL may be displayed in a label as follows:
Find it at : <%SitepagesUrl%>/Text with spaces.aspx
The behavior does not work as desired. If you put a page name with spaces, the “Find it at” field will generate %20 to replace the spaces:
This is due to calling encodeUriComponent, which is encoding the spaces to %20.