In FSBO [1] section 6 "Time Stamps", note that the LastWriteTime value gets updated when an IRP_MJ_FLUSH_BUFFERS is processed. In the Windows API, this is a FlushFileBuffers [2] call. In the C runtime, it's a _commit [3] call, which is an os.fsync [4] call in Python. Calling the latter will update the directory entry for the file. 

For an example implementation in the FAT32 filesystem, see FatCommonFlushBuffers [5]. Note in the UserFileOpen case that it flushes any cached data via FatFlushFile and then updates the directory entry from the file control block (FCB) via FatUpdateDirentFromFcb, and finally it  flushes the parent directory control blocks (DCBs) -- and possibly also the volume.

Example with os.fsync:

    import os
    import time
    import datetime

    UPDATE_DIR = True

    FILEPATH = 'C:/Temp/test/spam.txt'

    def scan(filepath):
        dir_path, filename = os.path.split(filepath)
        with os.scandir(dir_path) as iter_dir:
            for entry in iter_dir:
                if entry.name == filename:
                    return entry

    with open(FILEPATH, 'w') as f:
        while True:
            print('spam', file=f, flush=True)
            if UPDATE_DIR:
                os.fsync(f.fileno())
            entry = scan(FILEPATH)
            stat_result = entry.stat()
            now = datetime.datetime.now()
            print(f'st_mtime: {stat_result.st_mtime:0.3f}, '
                  f'delta_t: {now.timestamp() - stat_result.st_mtime:0.3f}')
            time.sleep(1.0)


[1] https://go.microsoft.com/fwlink/?LinkId=140636
[2] https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-flushfilebuffers
[3] https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/commit?view=vs-2019
[4] https://docs.python.org/3/library/os.html#os.fsync
[5] https://github.com/microsoft/Windows-driver-samples/blob/9afd93066dfd9db12f66099cf9ec44b6fd734b2d/filesys/fastfat/flush.c#L145