When the temperature on the dial is not set to the steam zone (2 dot or above), your iron will not be hot enough to steam. This can cause its soleplate to leak when you activate steam boost.
Please, make sure to set the temperature to the steaming zone on the dial (indicated in blue color in the image) before activating steam boost. Then, let iron heat up in heel-rest position for one minute before you resume ironing.
Note: The image is for illustration purposes only and may differ from the dial of specific models.