Well, the initial issue is likely because of FEATURE: Include optimized thumbnails for topics (#9215) 路 discourse/discourse@03818e6 路 GitHub where it seems thumbnail can be only uploads now. Discourse always first in breaking behavior/changes. Sorry for being salty, it鈥檚 just frustrating. 
I get it it now depends on Discourse behavior. Thanks for the help.
Just telling my status on this. Still struggling.
I鈥檝e 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鈥檝e already cleaned the html, no attributes except src). The criteria you鈥檙e 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鈥檝e extracted the HTML above is ignored. Whatever rake posts:rebake or in rails post.save! ; post.rebake! ; the topic is ignored. I can鈥檛 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