We describe atomic hydrogen diamagnetism within the framework of multichannel quantum-defect theory using an R-matrix approach. The calculated photoionization spectrum in the range of magnetic field B=${10}^{3}$--${10}^{4}$ T shows that the quasi-Landau resonances are broad interlopers that perturb high Rydberg states converging to the Landau thresholds, forming complex resonances. A partial-cross-section analysis indicates that electron population of different Landau channels depends on the azimuthal quantum number and parity of the final states. For odd-total-parity final states with m=1, the photoionized electron is predicted to escape predominantly in the higher Landau channels, while for final states with m=0, it escapes in the lower channels. This property is reflected in the shape of autoionizing resonances, which are more like peaks for m=1, but are more like dips (window resonances) for m=0.