If the I2C-Bus is not available for any reason 32 Bytes of free heap gets consumed each cycle..

[eXtr3me26]

This method needs to be changed:

bool Adafruit_MCP23XXX::begin_I2C(uint8_t i2c_addr, TwoWire *wire) {
i2c_dev = new Adafruit_I2CDevice(i2c_addr, wire);
return i2c_dev->begin();
}

into:

bool Adafruit_MCP23XXX::begin_I2C(uint8_t i2c_addr, TwoWire *wire) {
if (i2c_dev != nullptr) {
delete i2c_dev;
i2c_dev = nullptr;
}
i2c_dev = new Adafruit_I2CDevice(i2c_addr, wire);
return i2c_dev->begin();
}

Also, this seems to be a general problem of this library.. only a successful run seems to avoid free heap problems..