STM32L4 shutdown mode current consumption dependent on SYS_CLK

I am using the stm32L452RE Nucleo board and flashed the same with the shutdown mode example (default uses SYS_CLK = 80MHz) and observed the current consumption in shutdown mode to be in 500nA. But if the SYS_CLK is changed to 16MHz, 20MHz, 40MHz and flashed the current drawn is observed to be a constant 0.3 mA.

What might me the issue that is causing the shutdown mode to be dependent on SYS_CLK?