Before a Brown-Out Detector (BOD33) is enabled, it must be configured, as outlined by the following:

The BOD33 register is Enable-Protected (excluding BOD33.ENABLE bit), meaning that they can only be written when the BOD is disabled (BOD33.ENABLE=0 and STATUS.B33SRDY=0). As long as the Enable bit is '1', any writes to Enable-Protected registers (EVCTRL) will be discarded, and an APB error will be generated. The Enable bits are not Enable-Protected.