When we add a header in table using accessibility tab then we need that header should get wrap in <thead> and same for footer and rest of the rows will wrap in <tbody>. For details please go through video:
https://supportheroes.telerik.com/clientsfiles/d6c4fcb8-de14-4966-9bb4-bb0ff104b136_table-header-requirement-from-kendo-edited-mp4.zip

Steps to reproduce:

1. Navigate to this demo:
https://demos.telerik.com/kendo-ui/editor/index

2. Clean the contents

3. Create a table and click inside one of the cells

4. Open the Table Manager and open the Accessibility tab

5. Add 1 header row and click Ok:

The <th> cells go inside the <tbody> instead of <thead>: