Precise spectral characterization of every pulse is required in many x-ray free-electron laser (XFEL) experiments due to the fluctuating spectral content of self-amplified spontaneous emission (SASE) beams. Bent single-crystal spectrometers can provide sufficient spectral resolution to resolve the SASE spikes while also covering the full SASE bandwidth. However, strain and stresses due to the strong bending may affect the intrinsic diffraction properties of the crystal significantly and thus affect the spectral resolution of the device. This paper experimentally investigates the intrinsic diffraction properties of a strongly bent single-crystal diamond spectrometer to be implemented at the European XFEL and compares with theoretical calculations.