For PSoC 6 MCU, the RTC (WCO primarily) consumes around 380 nA typical and 1 µA max current; therefore, the power consumption of the whole BACKUP domain should not exceed the µA level. If a much higher value such as more than 100 µA is observed in XRES mode, it’s more likely to be caused by a crowbar current on P0_4.
P0_4 is one of the two wakeup pins for Hibernate mode, and thus the input buffer of P0_4 would always be enabled even under XRES mode because VBACKUP is the only power source for pins of port0. If P0_4 is left as floating, the crowbar current on it would consume much extra power. To eliminate the crowbar current, P0_4 must be tied to either PWR or GND. After doing that, the power consumption on the VBACKUP domain would be reduced significantly to a normal value.