In the URL and page title displays under “actions”, piwik treats the slash character “/” as a separator for the tree view that it generates. Apparently, if there is nothing to separate, it treats the “/” as part of the name, like in “/Cat1/” - while “/Cat1/Cat2” is split up properly. If that explanation is correct, you can try to things:
Don’t log names with a trailing slash.
Change the character that Piwik treats as a separator. The definition is in /config/global.ini.php in two params