MCP23017 connection to CD74HC4067 through Diode
I suspect you have the MCP23017 pin configured not as an output but as an input with a weak pullup and that you have an as-yet unreported 10K pulldown on the other side of the diode (on the input of the HC4067). 

1N400x are just regular very slow rectifier diodes, not Zener diodes. 

It's always better to supply an actual schematic, it prevents wasted time and guesses. If you use the schematic button you can edit your question to include a schematic.