Why does Ichigo Kurosaki's Zanpakutō (Zangetsu) transform back into its original form (shikai mode) when cutting Yhwach in half in the latest chapters of the Bleach manga (chapter 684)? Is it because it is the Ichigo's final bankai?
Before the final battle, Yhwach has absorbed Mimihagi, the right arm of the soul king, which governs stagnation (around 619). This means that Mimihagi is present during the final battle.
There are two mysteries within the final battle: how did Ichigo actually defeat Yhwach, when Yhwach could simply future shatter the sword, and why did Ichigo's sword regress to his original Shikai? The answer to both is Mimihagi. And we must answer the first before we can answer the second.
Mimihagi does not ever appear within Yhwach's future visions. We see Yhwach freaking out in 617 when Mimihagi intervenes. This makes sense: Yhwach can see the future, not the past.
After the Almighty fully activates, Yhwach's visions do not fail. Yet during the final battle, he gets shot by Uryu and cut by Ichigo. Obviously, he didn't foresee these events since they proved fatal. From what we know, we can deduce that Mimihagi is interfering with the powers of the Almighty. Yhwach cannot see visions involving Mimihagi, and since he absorbed Mimihagi, the Almighty will not behave perfectly when used on situations that he is in.
Before he is shot, Yhwach attempts to envelop the Soul Society in darkness. However, the power of Quincies isn't darkness. They wield weapons made of literal light. In employing Mimihagi's power so recklessly, he weakened the influence of the Almighty, allowing Uryu to fully disable it.
Now the answer to your question:
When Yhwach reached out his hand enshrouded in darkness to shatter Ichigo's blade, he accidentally employed the wrong power. He couldn't use the Almighty's power in that situation, so he inadvertently used Mimihagi. But Mimihagi doesn't destroy, it stagnates, it regresses. So instead of shattering Ichigo's blade, Yhwach forced it to return to the original form