The original zip file format only supports files up to 4 GB and 65,535 files. It looks like JUCE creates corrupt zip files if you create them too big, as it casts the 64 bit values to 32 bit and writes them to the file. I suppose this is fairly common and most zip programs can work around it, if you can extract your files.
There is a newer zip64 format that juce does not support which does allow for files over 4 GB and total archive size over 4 GB.
If you need to support zip files over 4 GB, look at using libzip https://libzip.org/