When do images get mirrored in the local instance's pictrs?

I'm running an instance for me and a couple of friends at https://lm.put.tf/. I've noticed that there seems to be no consistency whether or not post images are mirrored in instance's pictrs

For example:

The post https://lm.put.tf/post/22176 from !lemmyshitpost@lemmy.world has its image mirror from https://sh.itjust.works/pictrs/image/92ec8e81-1f05-4ff7-8ec7-f3bdee3d8087.jpeg to https://lm.put.tf/pictrs/image/747826a6-281f-4b1b-8ba2-7bbf452916dd.jpeg

However the post https://lm.put.tf/post/22060 from the same community, but posted by a user from lemmy.blahaj.zone does not have a mirror on my instance. The image links to https://lemmy.blahaj.zone/pictrs/image/OpIT86L1vq.jpg

Why is there a difference in behaviour? is it because the post was done from another instance and not lemmy.world? What is the replication/mirroring logic?

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I wish I knew.. I've seen the behavior you've described, but others tell me it doesn't happen.

I think I figured out the reason. Thumbnail previews are generated by your local instances, in the first post I've linked, the meme is actually a link to https://i.imgflip.com/7rgf1k.jpg which the instance downloads and generates a thumbnail for.

On posts that are actual images uploaded to the instance (e.g. the second link I posted), it looks like that lemmy just reuses the URLs.

Ok yeah, I guess for external links direct to images it makes sense for the image to be retrieved by each instance. Still, its not intuitive behaviour and it doesn't seem to be widely understood at all.

Thanks for reporting back.