The longitudinal structure of the quark-gluon plasma (QGP) remains poorly constrained. Using a transport-model calculation of two-particle elliptic-flow correlations $V_{2\Delta }(\Delta \eta )$ in relative pseudorapidity $\Delta \eta$, we find two distinct initial-state components of flow decorrelation: a long-range component driven by the global geometry of participating nucleons and a short-range component reflecting local energy-deposition fluctuations. This short-range component extends to $|\Delta \eta |\sim 4$ and can be misidentified as final-state nonflow. For isobar-like nuclei with different shapes, the calculation predicts an experimentally testable fingerprint of this component: it dilutes the deformation contribution at small $|\Delta \eta |$, so that the $V_{2\Delta }(\Delta \eta )$ ratio between deformed and near-spherical systems, such as U+U-to-Au+Au, rises with $|\Delta \eta |$. Separating long- and short-range flow decorrelations from nonflow gives access to the longitudinal initial structure and dynamics of the QGP.