Thanks! I’m using MEC Lite too, so that looks a likely contender to be the issue. I’ll try rolling back the version for now, but a long-term solution would be ideal, so I’ll follow that post!
UPDATE: I’ve figured out that this only happens when there is a manual (text) excerpt added to the post – if I leave the excerpt blank, then the featured image appears as it should on both the post page and the main blog page. However, this has the knock-on effect that the whole blog post is shown on the main blog page (rather than the excerpt I would have manually added), even when it’s set to show summary only in the WordPress settings.
I’ve removed my excerpts (and added them back as content) as a temporary workaround for the latest 12 posts (i.e. what shows on the first page of the main blog page), but if you look at the next page of posts you’ll still see the issue.
