In mid-to-late 2019, Creality upgraded the base Ender-5 leadscrew to the same one used on the Ender-5 Pro which prevents the bed from dropping once power is cut.
If your machine shipped with the upgraded leadscrew (or if your prints are 1/2 as tall as they should be), change your Z steps from 400 to 800 in Configuration.h:
Before:
DEFAULT_AXIS_STEPS_PER_UNIT { 80, 80, 400, 93 }
After:
DEFAULT_AXIS_STEPS_PER_UNIT { 80, 80, 800, 93 }
Or by sending M92 Z800 followed by M500 after updating your firmware.
After G28 add:
G29 ; auto bed leveling
M420 S1 ; Use mesh leveling