I first thought this bug might be due to the file size, our members often post 40MB+ images but I can replicate this issue on try.discourse.org using just a 5MB png image.
The image appears in the composer window when composing my post but does not appear in the post itself when submitted.
The image did appear briefly when I submitted that post but then it disappeared again a few seconds later, I should have done a screen recording. Is it being lost/broken in the baking process?
If I look at a post where the image has failed to be one boxed / pulled down a local copy, when I Edit the post I see this 404 Not Found error in the browser console:
I’ve been having another run at it today, but I’m afraid I am no closer to working out what’s happening.
When I try and add the https://dronescene.co.uk/images/uploads/fullsize/20211216142548_DJI_0205_HDR_Pano.png it shows in the preview, and in the post for a second, and then is just blank.
I don’t see any 404s. I did think it was down to the size, as it was loading line by line in the preview like it was the 90s But you said you tried with a smaller one and got the same. Which one was that, and I’ll try that one too.
I believe it’s removed because the file is too large. By default, the max image size kb is 4096kb, and this specific file is > 5000 kb.
From my understanding, if you upload an image, it’s automatically resized/compressed before sending. You can upload large photos because of the client-side browser optimization.
With download remote images to local setting, Discourse will try to download the image as it is, and if it’s too large, it will be removed from post content.
The raw content for this specific post (and the one below) on your forum shows that it is an uploaded image, not an external link. Because of Client side image optimization enabled by default, you will be able to upload large images.
Your initial issue is about inserting an external image link.
That said, I’m unsure if Discourse behavior is expected not checking the image size when pasting an image link.
Can you share the value of upload_size in your app.yml?
I don’t know about Discourse being able to download very large remote images.
However, I remember that previously, when you hotlinked a large image, it displayed a message like “This image is too large to be displayed in the forum”, with a link to the external picture.
i was able to reproduce this issue as well. i also went through all my image related settings to make sure nothing there was somehow causing the effect. i notice when i first make the post with large image link, it flashes for a minute like it’s going to load from the top of the photo, then just disappears (this despite the image being very visible in the post preview window).
i just tested in private messages and the image shows up fine as expected and links to the original. so the issue is strictly in topic posting views it appears.
But you said you tried with a smaller one and got the same. Which one was that, and I’ll try that one too. 
