  Cyclotron line scattering features are detected in a few tens of X-ray
pulsars (XRPs) and used as direct indicators of a strong magnetic field at the
surface of accreting neutron stars (NSs). In a few cases, cyclotron lines are
known to be variable with accretion luminosity of XRPs. It is accepted that the
observed variations of cyclotron line scattering features are related to
variations of geometry and dynamics of accretion flow above the magnetic poles
of a NS. A positive correlation between the line centroid energy and luminosity
is typical for sub-critical XRPs, where the accretion results in hot spots at
the magnetic poles. The negative correlation was proposed to be a specific
feature of bright super-critical XRPs, where radiation pressure supports
accretion columns above the stellar surface. Cyclotron line in spectra of
Be-transient X-ray pulsar GRO J1008-57 is detected at energies from $\sim 75
-90$ keV, the highest observed energy of cyclotron line feature in XRPs. We
report the peculiar relation of cyclotron line centroid energies with
luminosity in GRO J1008-57 during the Type II outburst in August 2017 observed
by Insight-HXMT. The cyclotron line energy was detected to be negatively
correlated with the luminosity at $3.2\times 10^{37}\,\ergs<L<4.2\times
10^{37}\,\ergs$, and positively correlated at $L\gtrsim 5\times
10^{37}\,\ergs$. We speculate that the observed peculiar behavior of a
cyclotron line would be due to variations of accretion channel geometry.
