Usually Postdoctoral Fellows do not have the Intellectual Property (IP) for the data they collect during their positions. When a Postdoc finishes their role, research institutions can potentially use their collected data for funding or publish it. In some cases, Ive seen manuscripts prevented from submission, and then 'reangled' by the PI with the PI then put as first-author, which is arguably unethical although probably legal.
Given a Postdoc is still actively working on the data after completion of their Postdoc position, at which point should they forfeit their 'rights' to be first-author, if ever?