only reload custom functions in dev

app/static/js/core/reload-custom-functions.js

@@ -1,5 +1,13 @@
 async function reloadCustomFunctions() {
   // Unofficial API: https://github.com/OfficeDev/office-js/issues/3486
+  // This causes issues when opening a workbook in a running instance of Excel. If
+  // Excel is completely closed, it does load the add-in, but not if it is already
+  // running.
+  // Custom functions will show #NAME? until the task pane is opened
+  // manually. That's why this is only loaded in "dev" environment. While
+  // await Office.addin.setStartupBehavior(Office.StartupBehavior.load); would solve
+  // the issue, it would make every workbook, which is opened while the add-in is
+  // installed, look for the add-in when reopened.
   await Office.onReady();
   let jsonMetadataString, code;
 

app/templates/base.html

@@ -54,7 +54,9 @@
       {# Load Custom Functions #}
       {% block custom_functions_code %}
         <script src="{{ url_for('custom_functions_code') }}" defer></script>
-        <script src="{{ url_for('static', path='/js/core/reload-custom-functions.js') }}" defer></script>
+        {% if settings.environment == "dev" %}
+          <script src="{{ url_for('static', path='/js/core/reload-custom-functions.js') }}" defer></script>
+        {% endif %}
       {% endblock custom_functions_code %}
       {# Bootstrap with the xlwings theme #}
       {% if settings.enable_bootstrap %}