This is a bug that has existed ever since the flying system was revamped with the BTU update.
Prior to 1.2, flying down into a block would completely cancel the flight. That was changed so that the only way of completely stopping flight is with a double jump. It was implemented so that the behavior would match the console edition's behavior.

What was expected
The flying speed would stay consistent as you descend into the block and let go of the crouch button.

What happens
Flying down into a block will momentarily "cancel" (you can still go up instantly after touching the block) your flight for a few seconds, giving you normal walking speed (and view bobbing). Footsteps can also be heard.