After a day of investigation, I'm very convinced that this .zip file is invalid: there's details over at https://github.com/itchio/butler/issues/142
However, in the process, I discovered that butler was rejecting other files that are valid, so I fixed that :)
I've added the `butler auditzip file.zip` command to make it easier to investigate those cases in the future.
For now, I'm really curious though - what software produced this file? What was your exact process? Because it's not even just a "rare variant", it's just plain wrong :)