If I export all my docs and try to re-import that same file, it works fine. However if I take a handful of those docs and copy them to a new folder, zip it, and re-upload, it says it can't find any documents.
I came about this while trying to debug errors in my import process which I initially thought was due to improper mapping of fields. The error described above however seems to indicate that maybe there's something wrong with the zip file itself?
I can share the docs & zip files with you if need be!
So I have investigated this issue and it turns out that it is a common issue.
in fact, If you have compressed a folder containing JSON files you'd like to import, the import won't work. You need to ZIP the files directly and not a folder.
I've tested this on a clone of your repo and it works, can you please confirm?