FFmpeg converts the WAV file from WAVE_FORMAT_PCM to WAVE_FORMAT_EXTENSIBLE when it considers the sample rate or width are out of spec.

Take a look at https://trac.ffmpeg.org/ticket/4426 for a discussion about it.

If you need a 96Khz WAVE_FORMAT_PCM file, FFmpeg will not produce it. Use audacity or DAW like reaper or cakewalk to generate it.