I use WinSCP for transfering files to various websites I manage, without any trouble for months.
I recently changed my Internet connection from ADLS to Fiber Optics.
I now face incomplete transfers of large files, e.g. this morning a 6.5Mb file was not correctly transfered. I tried to transfer it again to another location of the same site, and that time it was correct. In both cases the size of the remote file was the same as the local file, so there was no evidence that is has been truncated, but as the format of that file is specific to a web application, I asked the support of the application provider who detected the incompleteness of that file.
The wrong version of the file is available at <invalid hyperlink removed by admin>, the right one at <invalid hyperlink removed by admin>
Those links point to web pages. I'm not sure what to download. Can you post links to actual files?
Also you wrote that the files have the same size after upload. So what do you actually mean by claiming the files were "truncated"