We present the first measurement of the 47K(d, p gamma)48K transfer reaction, performed in inverse kinematics using a reaccelerated beam of 47K. The level scheme of 48K has been greatly extended, with nine new bound excited states identified and spectroscopic factors deduced. Uniquely, the 47K(d, p) reaction gives access to nuclear states that are sensitive to the interaction of protons and neutrons in the widely spaced 1s and fp orbitals, respectively. Detailed comparisons with SDPF-U and SDPF-MU shell-model calculations reveal a number of discrepancies between theory and experiment. Intriguingly, a systematic overestimation of spectroscopic factors and a poor reproduction of the energies for 1- states suggests that the mixing between the pi s11/2d43/2 and pi s21/2d33/2 proton configurations in 48K is not correctly described using current interactions, challenging our description of light nuclei around the N = 28 island of inversion.