Thanks for reporting this @arendvw. I looked into it and I found a case-sensitivity bug in the server-side response caching. The database is correct but the cache wasn’t invalidated so you got a stale response. I’ll fix this ASAP.
As a side note: from usability kind of view it would be nice to not be able to successfully yank a package twice: so there’s no confusion about if a package has been yanked.