I found the root cause of the problem:
I changed the slug for the post, and use a redirect from the old URL/slug to the new one. However, this redirect did not account for the presence of the AMP version, and requests for the AMP version were forwarded to the regular post.
I have set up a special redirect for the AMP version, and now AMP validator shows no errors for this post.
So this is a non-issue, as it was due to user error.