TinyMCE has a feature to insert predefined content into an article. It is accessed from the stamp icon in the tinyMCE toolbar. In the tinymce plugin options you define where the content templates are located ie which of you templates.

Steps to reproduce the issue

Create a content template (its just a text file with the file extension txt) in the html/tinymce folder of the cassiopeia template
Set that as the content template source in the plugin
Check that you can insert the content template in an article.

Now create a child template of cassiopeia and then open that newly created child in the template manager

Expected result

an empty folder template/cassiopeia_child/html/tinymce is created

Actual result

an empty folder media/templates/site/cassiopeia_child/html/tinymce is created

Additional comments

As a result of this if you save a content template into the folder provided it will not be found by tinyMCE