This error in datasheet tripped me up big time. It wasted me months of development time since it causes problem with the WiFi firmware. I'm posting it here so that others can avoid it.
In section 5.1 Pin Descriptions, pin 26 (VBAT_SIP) is described as "Power supply for backup when VDD_3V3_SIP is not present". That's not true.
1) That power supply input is mandatory in order for the 8Mb SPI Flash chip to get power. Without it the flash memory chip doesn't receive any power. And therefore the WiFi firmware was never properly downloaded into the flash memory, causing many problems.
2) That pin can ony accept an input between 0 and 3.6V (absolute max at 4V), making it unsuitable to directly tie to the battery input (therefore the signal name which suggests it's related to the battery voltage is misleading).