I no longer know what you're saying. Measuring a current, with a voltmeter? A W at a halfway point? Current appearing on power pins? Are you measuring current or voltage here? What's a "small voltage swing" in your opinion? Do you have a number to go with that, say, 1 volt? 3 volts? 5 volts?
You need to measure good, solid, 5V all the way to the actual MCP23017 power pins. Until you've actually verified that, you're wasting your time and mine.
I2C pullups should be between 4.7Kohm and 47Kohm.