It's not surprising it wasn't noticed. Routing follows the most specific route, so the cloudflare incident was obvious because Cloudflare annouced a /20 and somebody else announced a more specific /21. In this case it's the opposite, everything in 2400::/12 that's actually in use is already announced by a more specific route, somewhere around /29 to /48. The only affected addresses would be ones not in use.
Nobody noticed the unused addresses in a large IPv6 block going missing because they weren't in use so nobody was affected.