Render Template issue
The template path needs to include your plugin folder name. So in your case the path to your email templates would be pluginname/templates/emails/prospect and the path you pass to render() would be pluginname/emails/prospect.

More info: Plugin template paths explained

But that doesn't explain why the first template path appears to work.

I’d probably keep the templates inside my plugin if they weren’t being used from the front-end. However, if you'd like to change the default template path to your front-end template directory this answer will help.