Program crashes when calling external library code via function pointer

Question

I've written a user library for the Pi Pico that uses the TaskScheduler library to abstract timing away from the user. The user provides a function in their sketch which the library calls periodically via function pointer passed in the constructor.

When I stick to standard Arduino or functions from my library, everything works fine. If I include a function from the ADC library I'm using, core1 hangs while the comms on core0 continue to respond. If I run the same ADC operation from a non-callback function on the same core it works as expected. What is happening?

The relevant code snippets follow:

Library:

class Card {
public:
  typedef void (*ExtCbPtr)();  //pointer type to call sketch code from library
  ExtCbPtr _readCB;            //function pointer, points at user routine in sketch
  ExtCbPtr _writeCB;
Card(ExtCbPtr readFunc, ExtCbPtr writeFunc); //constructor
private:
  Task readInputs;
  Task writeOutputs;
  Scheduler ts1;
};
Card::Card(ExtCbPtr readFunc, ExtCbPtr writeFunc) {
  this->_readCB = readFunc;    //use to call user's sketch code
  this->_writeCB = writeFunc;
}
void Card::begin() {
  //init scheduler tasks
  ts1.init();
  ts1.addTask(readInputs);
  ts1.addTask(writeOutputs);
readInputs.setInterval(100);
  readInputs.setIterations(10);
  readInputs.setCallback(_readCB);
  writeOutputs.setInterval(100);
  writeOutputs.setIterations(10);
  writeOutputs.setCallback(_writeCB);
}
void Card::core1Loop() {
  ts1.execute();
}

Sketch:

static void writeOut();
static void readIn();
Card card(readIn, writeOut);
Adafruit_ADS1115 ads;  //ADC
void setup(){
rp2040.idleOtherCore();
//setup ADC
  if (!ads.begin()) {
    Serial.println("Failed to initialize ADS.");
  }
  ads.setDataRate(0x00E0);  //860 samples per second, every conversion takes ~1.2mS
  ads.setGain(GAIN_ONE);    //0.125mV/LSB
card.begin();  //initialise
  rp2040.resumeOtherCore();
  Serial.println("Ready 0");
}
void setup1(){
  delay(1);
  Serial.println("Ready 1");
}
void loop1();
  card.core1Loop();
}
void readIn() {
  Serial.println("Read start");
  uint16_t adc =  ads.readADC_SingleEnded(0);  //problematic line
  Serial.println("Read end");
}
void writeOut(){
   //Doesn't matter, task not reached if read fails
}