I have looked a bit around for the cause of these messages. In the first case I
guess I found a reason:
/*Driver went offline but SMU was still alive and contains the VFT table */
in line 340 in polaris10_smumgr.c

In case of the second message in line 575 in hwmgr.c the driver searches for
the maximum leakage voltage I guess, where the virtual voltage ID can have
values between 0xFF01 - 0xFF08, probably kind of representing the power states
ATOM_VIRTUAL_VOLTAGE_ID0 - ATOM_VIRTUAL_VOLTAGE_ID7.

But it doesn't find the specific voltage value in the table so instead of
setting the value of the *sclk destination it prints the assertion. That's kind
of my interpretation.
Unfortunately the rest is too intricate for me.

The messages disappeared with an R9 290X.

The RX 480-BIOS can be downloaded here:
https://www.techpowerup.com/vgabios/184548/xfx-rx480-8192-160614