LED display via MCP23017

Hi

I have a 16x16 led display connected to my Arduino Uno via a MCP23017-based IO expansion board and a DFRobot Gravity: I2C Multiplexer.

My sketch displays an image perfectly (all leds lit constantly) when the LED unit is attached to Arduino Uno pins directly.

However, when I move the connection to the MCP23017, the image leds turn on and off repeatedly.

Any suggestions?

Regards