SC16IS750 and higher baud rates

I'm using an SC16IS750 in an Arduino Mega project. It works fine up to a baud rate of 57600, but nothing higher. I can read and write data, it just comes out as garbage on the other end. Interestingly, if I set one of the Mega UARTs to 115200 and connect it to the SC16IS750 serial port, they communicate fine. That makes me think that the SC16IS750 is sending at 'approximately' 115200 but not exactly 115200.

Are there known issues with this part? Or am I just missing something?

Thanks

(Didn't include code because it works at all of the lower rates)