Capacitor's Filesystem.writeFile fails unless I specify encoding on iOS. Works on Android

I am trying to let users share a PDF, so I can't just specify the encoding as a workaround or share will share it as a textfile. Here's the code:

 function shareFile(fileName, base64Data) {
 
   return Filesystem.writeFile({
      path: fileName,
      data: base64Data,
      directory: Directory.Cache,
      // encoding: Encoding.UTF8   <-- this line fixes writeFile but 
      //                               breaks share
      recursive: true
    })
      .then(() => {
        return Filesystem.getUri({
          directory: Directory.Cache,
          path: fileName
        });
      })
      .then((uriResult) => {
        return Share.share({
          title: fileName,
          text: fileName,
          url: uriResult.uri,
          dialogTitle: fileName
        });
      });

I've tested with multiple PDF files. They all work on android but fail on all versions of ios I have tried. If I let it encode as UTF8, the shared file is just a shared utf8 encoded base64 string instead of a PDF.