On the WB50NBT, is it absolutely mandatory to feed VCC3_3 and VDD_BU from different power supplies?

Answer

On the WB50NBT both VCC3_3 and VDD_BU can be powered from the same supply. However the real time clock and SRAM registers are tied to VDD_BU. So with these lines tied together and the 3.3v supply is removed the module need to from scratch each time.