RP2040 Core1 stops when connected via usb

after trying to fiddle with the pi pico trying to get core1 to "fade" the builtin led (pin 25 ) it would stop right after upload.
thinking it was a voltage issue (_unlikely but who knows_) i tried running it without usb and giving it 5v on Vusb directly, then it _did_ work.

(i already wrote something similar in a different issue but that was related to something else)
https://github.com/arduino/ArduinoCore-mbed/issues/217#issuecomment-1004352674

the code i used:
```
#include
#include
#include "pico/multicore.h"
#include "hardware/structs/sio.h"
#define CORE1_LED 25u
int brightness=0;
int fadevalue = 10;
void core1_loop()
{
while(1)
{
brightness=brightness+fadevalue;
if(brightness <=0 || brightness >= 2048) {
fadevalue = -fadevalue;
}
digitalWrite(CORE1_LED,HIGH);
sleep_us(brightness);
digitalWrite(CORE1_LED,LOW);
sleep_us(1000);
}
}

void setup()
{
pinMode(CORE1_LED,OUTPUT);
//multicore_reset_core1();
multicore_launch_core1(core1_loop);
}

void loop()
{
}
```

Start by reproducing the supplied sketch through setup(), core1_loop(), and loop() with the RP2040 connected by USB, then compare it with the reported 5V Vusb behavior. Trace the ArduinoCore-mbed RP2040 multicore and USB entry points involved in that sketch. Done means core1 continues running and the built-in LED fades while USB remains connected.

Written by the indexing model from the issue text.