This is not something intended, as you cannot turn it off in the settings when using the action buttons. When playing in Creative mode, do these steps to reproduce this bug:

Step 1- Change control mode to "Joystick & aim crosshair";

Step 2- Enable "Show Action Buttons" if not already enabled;

Step 3- Enter or create a world with your game mode set to Creative;

Step 4- Try to break any block.

Expected Result

The block is instantly broken.

Observed Result

The block will disappear for a split second, but will not break unless you hold the Attack button.