LastWriteTime isn't updating on write because Window's Buffered I/O and File Caching prioritize file content over metadata.
Metadata is only updated when a file is closed, flushed, or a user or system action (ex. read, disk write) forces a cache refresh.

For your reading pleasure:
Windows File Caching
Windows Buffered I/O

Is this sane? Not for continuous write or real-time analytics. That's why such systems don't use NTFS.