@Seldaek the tar file that you create somehow looks off.

The generated tar file with the composer archive wpackagist-plugin/individual-multisite-author 1.0 -vvv command from the issue cannot be opened.

Opening it with new \PharData($file) throws Uncaught UnexpectedValueException: Cannot create phar 'wpackagist-plugin-individual-multisite-author-1.0-a42e54.tar', file extension (or combination) not recognised or the directory does not exist. Extracting it with tar seems to work

tar -xvf  wpackagist-plugin-individual-multisite-author-1.0-a42e54.tar
tar: A lone zero block at 1

Ok can you try with the latest commit?

Works! No warnings/notices anymore with the tar command too!

Ok great 👍🏻