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Yeah, this is at least part of the problem. The bundle that Itch downloads appears to be corrupt beyond that, but with the manual download of the .rar, I was able to replace the JRE in the bundle with  a JRE download from https://www.oracle.com/technetwork/java/javase/downloads/jre8-downloads-2133155....  and it ran

That makes sense. The packaging tool we're using strips down the JRE to reduce download size, and it seems it was overzealous. I'll see if I can tweak its settings to minimize the JRE correctly, and failing that, package the full JRE for the macOS build.