How to turn-off the heuristic guessing of (kanji + okurigana) while doing Japanese input?
That is indeed a new feature for El Capitan:

Mac becomes even more fluent.
Japanese
Live conversion for keyboard input.
OS X El Capitan dramatically improves the ease and speed of entering Japanese text. With an enhanced vocabulary and improved language engine, it automatically transforms Hiragana into written Japanese as you type — eliminating the need to press the space bar for individual word conversions.

You can disable Live Conversion under System Preferences &gt; Keyboard &gt; Input Sources &gt; Japanese, or the shortcut in the Input Menu Item by clicking Open Japanese Preferences: