The Explanatory Memorandum and the Advice of the Council of State are always attached to a Bill. In the Memorandum can be read that the practice as developed since 1900 needed codification in the legislation.
In the Memorandum is explained that the King can not be Prince (Princess) of the Netherlands. The two titles bite each other. When in the Act is stated that the presumed successor of the King, as well the King who has abdicated the kingship bear the title Prince (Princess) of the Netherlands, then this is clear.
Add to this the examples set by three consecutive predecessors of the current King, then King Willem-Alexander will revert to the style he had before being the King or the Heir. Like his mother Beatrix, his grandmother Juliana and his great-grandmother Wilhelmina: they form "the existing practice which needs codification in the legislation".