Transition metal dichalcogenide heterostructures host long-lived interlayer excitons in the +K and -K valleys that can encode long-lived valley polarization. We investigated the low-temperature valley polarization dynamics in an MoSe2-bilayer hBN-WSe2 heterostructure. The bilayer hBN spacer results in an interlayer exciton lifetime exceeding 2 μs and an anomalous valley polarization amplification effect. Our theoretical model shows that this amplification results from the long exciton lifetime and exciton-exciton interactions. This work sets the stage for long-lived interlayer excitons to be used in nonlinear valleytronic transistors and amplifiers.