I'm going to have to spend more time to analyse this (later), but it seems like Windows deciding not to update the directory's data structures (containing the st_mtime retrieved by scandir) as long as the file is still open.

I suspect the answer for your scenario is that you'll just have to use os.stat() to get the information from the file's entry, rather than the directory's entry. It's unlikely there's anything we can do at Python's level without sacrificing all the performance gains of scandir() for all other scenarios.