Some files are not listed?!

When listing the content of a directory, then downloading the full directory, some files are NOT included! We saw this when copying some pics. Pics are available by their URL but are not visible in the directory through the plugin. And are not included in the ZIP file that is downloaded.
Concretely it concerns uploads/2021/03 there are about 2100 files. At least one .jpeg is not listed/downloaded.