MIUI 12 heavy battery drain is a known issue, fix may come in future builds
MIUI 12 update battery drain hitch continues to affect phones like the POCO F1, Redmi Note 8 Pro, and Xiaomi Mi 9 with no official fix in sight - NotebookCheck.net News
Mi 9][MIUI 12] Extreme battery drain on Standby | Xiaomi European Community | MIUI ROM Since 2010