i think folks using iNatForum as a generic image host has never really been an intended use. to the extent that folks use it this way, they would be subject to the whims of the iNatForum setup, which could change at any time for any reason without notice.
iNatForum probably uses CDN caching for static assets. so probably your images in the old location are still cached in your particular node of the content delivery network. but the cache gets cleared at some point. so even for folks in your same node, the images from the old location probably won’t work for long.
i would guess there’s not a bug here. i suspect it’s just the potential consequences of unsanctioned usage catching up with you.