That's an interesting bug. It is actually the first image that is causing problems:
https://media.8ch.net/cuteboys/src/1436731548308-0.webm
After trying it once, hydrus did import it but didn't send it to the page. When I run the thread checker again, hydrus attempts to redownload the file, sees the redundancy, and still doesn't send it. The re-download suggests that the md5 hash isn't being calculated right (and hence matched) somewhere. I calculated it manually, and 8chan reports it correctly, so perhaps hydrus is storing it wrong or something. I am not sure why it would not report to the page correctly–it does for a manual hdd import, so I guess the thread checker isn't expecting to have a redundant file for which the hash doesn't match? Not sure.
I'll look into it when I am back at my dev machine. Thank you for reporting this.
Also, in looking at a bit of 8chan's json api just now, I see they report additional images now, so I will amend my 8chan thread checker to parse multi-file posts!