From the new code (first paragraph is from the OT Devanagari spec.):

  /*   o Reorder matras:
   *
   *     If a pre-base matra character had been reordered before applying basic
   *     features, the glyph can be moved closer to the main consonant based on
   *     whether half-forms had been formed. Actual position for the matra is
   *     defined as “after last standalone halant glyph, after initial matra
   *     position and before the main consonant”. If ZWJ or ZWNJ follow this
   *     halant, position is moved after it.
   *
   * IMPLEMENTATION NOTES:
   *
   * It looks like the last sentence is wrong.  Testing, with Windows 7 Uniscribe
   * and Devanagari shows that the behavior is best described as:
   *
   * "If ZWJ follows this halant, matra is NOT repositioned after this halant.
   *  If ZWNJ follows this halant, position is moved after it."
   *
   * Test case, with Adobe Devanagari or Nirmala UI:
   *
   *   U+091F,U+094D,U+200C,U+092F,U+093F
   *   (Matra moves to the middle, after ZWNJ.)
   *
   *   U+091F,U+094D,U+200D,U+092F,U+093F
   *   (Matra does NOT move, stays to the left.)

Fixes https://github.com/harfbuzz/harfbuzz/issues/1070

Test case added with Adobe Devanagari.