Setting (digitalWrite) pin y, unsets pin x

- Arduino board: Custom board

- Arduino IDE version (found in Arduino -> About Arduino menu): PlatformIO

- List the steps to reproduce the problem below:
Run below sketch, it is expected that pin5 is toggling every 2 seconds.
It will however prematurely turn off setting another pin.

```
#include
#include

Adafruit_MCP23X08 mcp;
TwoWire my_wire(0);

void setup()
{
Serial.begin(460800);

my_wire.setPins(5,6);
mcp.begin_I2C(0x27, &my_wire);
mcp.pinMode(5, OUTPUT);
mcp.pinMode(6, OUTPUT);
}

void loop()
{
delay(2000);
digitalWrite(1, HIGH);
mcp.digitalWrite(6,LOW);

delay(2000);
digitalWrite(1, LOW);
mcp.digitalWrite(6,HIGH);

mcp.digitalWrite(5,HIGH); //This write also sets pin6 low ???
delay(50);
mcp.digitalWrite(5,LOW);
}
```

Start with the provided sketch and reproduce the interaction between digitalWrite(1) and Adafruit_MCP23X08::digitalWrite(5,6). Read the library implementation and its pinMode/state handling to determine why writing pin 5 changes pin 6. Done means the reproduction no longer changes pin 6 unexpectedly, with the behavior verified on the reported setup.

Written by the indexing model from the issue text.