Liquid-metal embrittlement (LME) by Zinc (Zn) is a prominent cause of early failure for advanced high-strength steels (AHSSs). To uncover the fundamental origins of this failure mode has been a challenge due to the fast, localized, and high-temperature process during typical conditions where LME occurs. Instead, a phenomenological understanding based on LME-crack studies has been a focus, with detailed investigations of micron-scale microstructural phenomena and mitigation at the processing level. To assess and quantify the first microstructural mechanisms during the very early stages of LME, we pursue here interrupted welding of a galvanized AHSS and focus on the evolution of the grain-boundary structure using scanning transmission electron microscope (Mat. Tod. Adv. 13 (2022) 100196). At temperatures well below the ductility trough, significant Zn enrichment and grain-boundary phase formation is observed. In concert with CALPHAD-integrated density-based phase field modeling (Scripta Materialia 238 (2024) 115758), we reveal how even small amounts of Zn can segregate and drive intermetallic phase formation, which finds its origin in a segregation transition of the Fe-Zn couple (Acta Materialia 259 (2023) 119243). These findings direct focus onto LME-controlling microstructural and thermodynamic phenomena at temperatures below the ductility trough and the austenite formation temperature.