Hi Matthew,
Kindly check if the external power down pin was latched high when interfacing to the microcontroller if you are using such pin. In addition whenever you issue the IO_UPDATE, try making its width as wide as possible. I have encountered these things before using the eval board and I just fixed it when I checked those that I mentioned.
Don't worry about the signal having a triangular shape as long as it has the correct frequency. We do setup it in the lab and we got same results with you.
Best Regards
Louijie