<p>Quantum magnonics is an emerging field of research with great potential for applications in quantum technologies, including quantum computing, information processing, and coding. Of particular interest is magnon Bose–Einstein condensation, which can occur even at room temperature due to the small mass of magnons and their weak interaction even at high concentrations. The article describes unique experimental results obtained in the study of the phase transition from magnon gas to Bose condensation with increasing magnon density. It is shown that the properties of a magnon gas are well described by the quasi-classical Landau–Lifshitz–Gilbert equations. However, upon transition to the quantum state of Bose–Einstein condensation, the properties of magnetization precession change dramatically. Instead of spin waves formation outside of the magnon generation region, coherent precession of magnetization occurs. In this paper, attention is drawn for the first time to the change in the direction of phase gradients at the boundary of the magnon excitation region and an explanation of this effect is given. The experiments were carried out in a Yttrium Iron Garnet film under the continuous RF pumping.</p>