In short, unlawful means different things in different contexts.

In the context of false imprisonment, it generally means without legal process, and legal process later overturned does not count.

See eg. https://caselaw.findlaw.com/court/us-supreme-court/549/384.h...

>Reflective of the fact that false imprisonment consists of detention without legal process, a false imprisonment ends once the victim becomes held pursuant to such process--when, for example, he is bound over by a magistrate or arraigned on charges. Dobbs, supra, §39, at 74, n. 2; Keeton, supra, §119, at 888; H. Stephen, Actions for Malicious Prosecution 120-123 (1888). Thereafter, unlawful detention forms part of the damages for the "entirely distinct" tort of malicious prosecution, which remedies detention accompanied, not by absence of legal process, but by wrongful institution of legal process