On this page https://www.akeeba.com/documentation/admin-tools-joomla/cleantmp.html is described what the Clean Temp-directory Button in Akeeba Backup does.
Quote: "...it will automatically remove all files and directories from your Temp-directory except index.html, index.htm, web.config and .htaccess, if any of those files exists."
But it DOES delete the index.html file. Is that a problem or a bug or can it be ignored because it is unimportant?