Imgur blocks access to most VPN IP addresses so any posts using the host will result in many users being unable to see the content. Sometimes only remote viewing is affected but is also often blocked when trying to open in new tab and view at the parent site.
Consider uploading to the fediverse or using a privacy-respecting hosting service like postimages.
I don’t know how common that is across different instances, but it’s definitely not just for comments. I often find myself looking up some old post that I had created using a Catbox link, and when I look it up from that account’s instance Lemmy.One, it’s the canonical Catbox link. But on Lemmy.World, the image link for that post is using a proxy.