Okay so it looks like there's a race condition in the "destroy old servers" system, where the image is being held open because it's linked to an instance that no longer exists.
@aurynn Mastodon's ratelimits are very restrictive, even the normal (mobile) webclient runs into them quite a lot. You can check the headers to see how quick you're going through the 300 requests / 5 minutes
@aurynn I have a patch for the authenticated users' ratelimit but might not be ideal for you