<p>Magnon, the quanta of spin wave, low energy excitation from magnetic ground state, not only carries spin angular momentum which is of crucial importance in new generation of information technology, but also serves as powerful probes for investigating the corresponding ground-state properties. Here, we investigate magnetic order transitions in the antiferromagnetic van der Waals insulator NiPS₃ using non-local magnon transport. We observe a dimensional cross-over behavior with a critical thickness of approximately 12–14 nm. Below the threshold, the thermally activated magnon carries angular momentum that is opposite to the conventional case, corresponding to the vestigial order with higher symmetry. While above this critical thickness, where NiPS₃ exhibits in-plane zigzag antiferromagnetic order with lower symmetry, the thermally activated magnon signals show anomalous high-magnetic-field responses. After the spin-flop transition, the Néel vector becomes strongly pinned near the <i>a</i>-axis, resulting in a flattening of the detected signals that can only be switched when the magnetic field is oriented perpendicular to the Néel vector. These findings demonstrate that magnon spin currents provide an effective means to investigate exotic orders and phase transitions in van der Waals magnetic insulators, offering new insights for both fundamental research and potential applications in spin-based technologies.</p>