Surface Pro 12 battery drains when shut down

Because the charge drops after an explicit shutdown and Wi-Fi works only after a second boot, the remaining evidence points beyond ordinary sleep settings—possibly firmware, a device power-state problem, or hardware leakage. Before service, install Surface firmware and driver updates, including optional Windows updates, then run Surface Diagnostic Toolkit. Generate powercfg /systempowerreport and /sleepstudy; confirm whether the interval is Shutdown rather than Modern Standby. Record the battery percentage, run shutdown /s /f /t 0, disconnect USB devices and the keyboard, wait 24 hours, and record it again. Repeat once with Wi-Fi disabled in UEFI. If drain persists while the report shows S5 shutdown, Windows applications are not running and another reset is unlikely to help. Save both reports and test results for support. The Wi-Fi failure after cold boot strengthens the case for firmware or hardware servicing, so accept the engineering inspection rather than changing more registry or power settings.