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

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..

Start at Adafruit_MCP23XXX::begin_I2C and reproduce repeated initialization attempts with the I2C bus unavailable. Verify that failed cycles no longer consume heap, and check whether the same ownership problem appears in other initialization paths in the library.

Written by the indexing model from the issue text.