  We report on analysis of observations of the bright transient X-ray pulsar
\src obtained during its 2017-2018 giant outburst with Insight-HXMT,
\emph{NuSTAR}, and \textit{Swift} observatories. We focus on the discovery of a
sharp state transition of the timing and spectral properties of the source at
super-Eddington accretion rates, which we associate with the transition of the
accretion disk to a radiation pressure dominated (RPD) state, the first ever
directly observed for magnetized neutron star. This transition occurs at
slightly higher luminosity compared to already reported transition of the
source from sub- to super-critical accretion regime associate with onset of an
accretion column. We argue that this scenario can only be realized for
comparatively weakly magnetized neutron star, not dissimilar to other
ultra-luminous X-ray pulsars (ULPs), which accrete at similar rates. Further
evidence for this conclusion is provided by the non-detection of the transition
to the propeller state in quiescence which strongly implies compact
magnetosphere and thus rules out magnetar-like fields.
