try apple-touch-icon

.gitignore

@@ -24,3 +24,9 @@ instance
 
 # Environment files (might contain secrets)
 .env
+
+# weird macOS files (maximum compatibility)
+.DS_Store
+._.DS_Store
+**/.DS_Store
+**/._.DS_Store

portal/templates/index.html

@@ -10,7 +10,11 @@
         />
         <link
             rel="icon"
-            href="{{ url_for('static', filename='img/portal.svg') }}"
+            href="{{ url_for('static', filename='static/img/portal.svg') }}"
+        />
+        <link
+            rel="apple-touch-icon"
+            href="{{ url_for('static', filename='static/img/portal.png') }}"
         />
     </head>