What can prevent entering STOP mode? (STM32WB)

[OliM]

After some activity using BLE in the background and UART as well as some EXTI on application side, the consumption jumps from 2,4µA to 4mA, so obviously I have a wakelock. If I hotplug to the system afterwards, I can see in PWR_EXTSCR that Stop mode is no longer actually reached (C1STOPF is not set). But checking ISPR I can not see any pending ISRS (the one after Stop is the LPTIM of my OS).

What else can prevent the CPU from entering stop? Which registers should I check?