Clicking "Automatically copy over from HTML" or "Copy from HTML" in the Email Editor fails to copy the contents over to the Text Only version of the email.
This can occur if there are no editable sections in the email asset. Only in editable sections of an email will be copied over to the Text version of an email.
Here is additional documentation: https://experienceleague.adobe.com/docs/marketo/using/product-docs/email-marketing/general/creating-...
A quick workaround would be to manually copy and paste the content you wish in the Text Version, and in the future build/use templates to make emails with editable sections.