Low-power Sleep doesn’t work after programming - SAMD21

I have an issue with my SAMD21 board. The board consumes around 40uA in SLEEPDEEP mode. Only, after reprogramming the board, it consumes around 9mA. A lot more... The reset cause after reprogramming the board is 64, System Reset Request.

The low power sleep code:

```
setup(){
// Sets SLEEPDEEP
SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;
}

loop(){
// Put SAMD in sleep
__WFI()
}
```

The board goes back in sleep when the battery is unplugged/plugged back in. I can't figure out why the board doesn't sleep after programming.

Any help?

Start by reproducing the SAMD21 power difference with the shown setup() and loop() code, comparing SLEEPDEEP and __WFI() behavior after programming versus a battery reconnect. Inspect the reset-cause value of 64 and determine what state differs between those startup paths; done means the cause is identified and the post-programming sleep current matches the expected low-power behavior.

Written by the indexing model from the issue text.