A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Unlike other types of placeholder & text boxes, Title Placeholders are designed to not contain a large body of text. Even though the Shrink on Overflow property is the default there is a 4 pt limit on the reduction. Once that limit is reached additional content is allowed to overflow the placeholder.
The workaround is to set the font size for the Title Placeholder to be significantly smaller but IMHO that's a practice which should be avoided.