Same problem here with my RX480. Even with driver 17.7.2, which I used before 'cause playing Overwatch the problem exists - even until 17.12.2.
For me it's always 665MHz. Even after clean uninstalling with DDU the problem exists nearly every second reboot. So if I correct the clock it's okay and working, after a reboot it may be set at default value but it also can be reset to 665MHz.
Setting it to another value like 1335 MHz doesn't fix it either, every other value is stored correctly, resetting the profile also doesn't help. So it should be a bug which exists very long.
System: AMD Ryzen 7 1700X // ASUS PRIME X370-PRO // 32GB RAM // PowerColor RX 480 // Windows 7