MathML not accepted as valid content for th element

Ace 1.3.2, Ace by DAISY app

The error was Minor, Best Practice, empty-table-header with the text

> - Element does not have text that is visible to screen readers
> - Ensures table headers have discernible text
> - Table header text should not be empty

The content of this `th` element was

```html

```

It appears that the MathML is being read as no text or as inaccessible text. Is this error correct? Should MathML not be used in `th`?

Thank you for any assistance that can be provided.