The transient mixing dynamics of an initially segregated binary granular system in a half-filled rotating drum are experimentally investigated. The granular system consists of spherical beads having identical size. The density ratio between the two granular phases is 2.8. With its ability to scan three-dimensional opaque systems with a high frequency, the ultrafast X-ray computed tomography is used to capture the transient and steady-state segregation dynamics in the bulk. The segregation dynamics are also compared to those at the circular end-wall caps, which have been captured with a camera. The results show an axial migration of the denser particles towards the bulk and, more importantly, second-order overshooting dynamics in the radial mixing index, which tend to increase with the Froude number. The results will find application in industrial systems, where rapid mixing occurs. We also believe the presented data can serve as validation for future three-dimensional simulations focusing on the transient formation of segregation patterns in the bulk.