According to this review it uses a PWM driver to control the 3 modes
So that means there is a transistor (likely a FET) on the board. There may be reverse protection also but i haven’t seen the board to tell. It is possible that the FET has a fault and it is draining the cell.
does the light still work using other cells/cell types, etc?
[edit] the 0.528 volts is typical of a diode drop; most FETS have a body diode built in. It could be that is the reason it didn’t go to zero—the diode would quit conducting below the drop voltage.