Well, the initial issue is likely because of https://github.com/discourse/discourse/commit/03818e642a1ae871bffdc0c39c10f05f0b8b0398 where it seems thumbnail can be only uploads now. Discourse always first in breaking behavior/changes. Sorry for being salty, it’s just frustrating. 
I get it it now depends on Discourse behavior. Thanks for the help.
Just telling my status on this. Still struggling.
I’ve tried to normalize img html to include only src attribute since it seems to be ignored by discourse. For example usually a post from WP would start to contain this (here I’ve already cleaned the html, no attributes except src). The criteria you’re talking about does not seems to be related.
<div data-wp><a href="https://www.aliexpress.com/item/4000096717124.html" target="_blank"><img src="https://zuzu.deals/wp-content/uploads/2020/05/5ebd241f6e871-150x150.jpg" /></a><div><div data-buy>
Then rebaking (so image are downloaded) from the console would work only for some topics.
Typically the topic where I’ve extracted the HTML above is ignored. Whatever rake posts:rebake or in rails post.save! ; post.rebake! ; the topic is ignored. I can’t see a reason why it fails, it has the same html as the others topics which worked.
However, if I edit manually from composer without changing the content and saving, then the image is extracted/downloaded. There are too much topics to edit manually.
Do you have an idea how you could achieve the same saving as composer does with rails console?
Note: the max old days option is set to the maximum value