We report a high magnetic field study up to 55 T of the parent and the nearly optimally doped iron-pnictide superconductor Ca 10 ( Pt 3 As 8 ) ( ( Fe 1 − x Pt x ) 2 As 2 ) 5 [ x = 0 and 0.078(6)] using magnetic torque, tunnel diode oscillator technique, and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near T c and signatures of multiband superconductivity. Unexpectedly, we find a prominent anomaly in magnetic torque close to 22 T, when the magnetic field is applied perpendicular to the ( a b ) planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. We suggest that this field-induced transition, observed both in the magnetically ordered parent compound and a nonordered superconducting sample, is a signature of a spin-flop-like transition associated not with long-range order but driven by antiferromagnetic fluctuations of magnetic moments aligned preferentially out of the conducting planes at low temperatures.