SFTP upload repeats the tail of large files

[littlewatchman]

upload of small files seems to work fine.
for some longer html I find the last few lines repeated on the uploaded file.
for a big zip file, 66.4Mb .... the uploaded version is 68.8Mb and won't unzip. But if I download it and manually truncate it at the correct length, it unzips and is 100% correct content.

so clearly it seems to be just repeating bytes to the tail during upload.

my upload is pretty basic
sftp.upload(data: data, remotePath: remote, permissions: .default)

so not sure how it can be something I'm doing wrong, but just in case, please let me know.
otherwise I'll download the source code and see if I can find out what's going on.

[littlewatchman]

BTW... I also did try saving the data to a local file and using
sftp.upload(localURL: localURL, remotePath: remote, permissions: .default)
exactly the same problem, creating exactly the same remote file. 100% identical, including the same extra bytes at the end

[littlewatchman]

code looks right (actually looks good too). stepping through ended with bytes sent being correct.
might be at the ISP end? so weird.
but if anyone has suggestions... thanks in advance