Only show the subtitle if it's an article, not a tag, series, or category list, by checking if the date and author are also set.

layouts/_default/list.html

@@ -21,7 +21,7 @@
                             <a href="{{.Permalink}}" class="post-item-inner">
                                 <span class="post-title">
                                     {{ if .Params.tags }}{{ index .Params.tags 0 | title }}:{{ end }}
-                                    {{.Title}}{{ if .Params.subtitle }}
+                                    {{ .Title }}{{ if and (.Params.subtitle) (and (.Params.author) (.Params.date)) }}
                                         {{ "--" | markdownify }} {{ .Params.subtitle}}
                                     {{ end }}
                                 </span>