An ongoing challenge in the study of quantum materials, is to reveal and explain collective quantum effects in spin systems where interactions between different modes types are important. Here we approach this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an easy-plane quantum magnet near a continuous quantum phase transition. Our inelastic neutron scattering measurements of Ba2FeSi2O7 reveal the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. The decay of the longitudinal mode is particularly pronounced at the zone center. To explain these observations, we develop a generalized linear spin-wave theory, including all of the one-loop corrections, which reproduces the measured mode decay and renormalization. The theoretical approach developed here is broadly applicable to quantum magnets with more than one type of low energy mode.