Yerr I see it now for categories – I get similar issues on one of my sites. There are difficulties for both blog posting pages and categories. It is not easy to solve either because of the variations which are possible.
With blog posting pages the type ‘WebPage’ is being used and in this case a default image is picked up and used. The page therefore validates but this is in fact not totally correct if you look at a well designed canonical page. For example, the way Genesis framework does it is to use ‘CreativeWork’ type. This is how the structured data should look for such a page
https://search.google.com/structured-data/testing-tool#url=https%3A%2F%2Fdemo.studiopress.com%2Fgenesis-sample%2F
The problem comes that if you do what Genesis does then there is no default image. An extra Webpage attribute would need to be defined with a default image.
Basically, blog pages were not conceived as having AMP versions – they don’t have a featured image like a blog post or web page sitting at the top.
With categories the same problem exists. However, it is not rendering WebPage with a default image – therefore it will not validate.
It is worth remembering that Google has been clear that structured data whilst welcome is not mandatory for AMP pages. This problem shouldn’t be effecting AMP validation for example.