Thanks all for the feedback. I updated the original post, added the schematic, and added some code.
Right now, I do not have anything connected on the other side of the bus. It's simply the uC -> MUX -> the buffer chip. I'm just trying to open and close the channel without a hangup. I can open the channel fine, but when I close it or make calls to read the sensor (once hooked up) is when the uC hangs.
I'm using a phone cable that is around 3 feet to communicate with an I2C sensor (LUX sensor from Adafruit). No other circuits.
It's simply the uC -> MUX -> buffer on starting point -> buffer on endpoint -> slave. Right now, I only have the uC -> MUX -> buffer on starting point connected for testing, but I get the same result even after connecting the LUX sensor.
I was incorrect on that statement and updated the description. It's specifically when I am trying to close the channel or read the sensor from the MUX is when it hangs. Wire.endTransfer() is successful when opening the channel.
Yes. uC -> buffer -> sensor works.