In Roadmap to HTTPS: serving and uploading HTTPS-images only, Step 3 said:
After this is enabled, old posts that have HTTP images, accessible via HTTPS, will be edited accordingly. This will be an actual markdown change, attributed to the Community user, posts won't be bumped.
Today, I revisted an old post I'd made, which had images from HTTP URLs that were part of StackExchange, and found that the images were no longer displayed but instead replaced by text URLs. I had to edit the post manually to add an
http and that worked just fine.
This change should have been made automatically. That it was not, indicates some bug or incompleteness in the script which adjusted image URLs. That bug may affect other posts too.