It turns out that Codex returns the image URLs in a way that is technically not part of the OPDS 1.2 specs. We used to support that, but Komga’s creator explained it’s not part of the official specs. We decided to remove the support without knowing Codex used that approach. That’s why the thumbnails stopped showing up.
Given that it doesn’t affect the rest of the servers, and it’s an addition to the specs, we have decided to bring it back so Codex works correctly.
We will try getting thumbnail support for folders into the OPDS 2.0 spec, which is currently a Draft.
Bottom line: the next version of Panels will bring back the thumbnails on Codex folders.