When you connect your external 10 MHz reference signal, do you disconnect the onboard 25 MHz reference by removing R59 and L10? Otherwise the two reference signals will cause trouble.
The standard loop filter on the evaluation board is designed for a 25 MHz PFD frequency. You are using a 10 MHz PFD frequency so this will change the loop filter dynamic. You can counteract this by enabling the reference doubler (Register 2) to generate a 20 MHz PFD frequency, and/or change the charge pump current to change the loop filter dynamic.