It seems source:volt? always returns the fixed value and not the value generated by the funcgen if it is active.
Example in simulator:

How to reproduce:
Set the value of AOUT and remember it. Set the funcgen on on AOUT with a range outside of the previous value. Query the AOUT value with source:volt? It will give back the first set value and not the actual current value produced by the funcgen.
If I do the same on DCP405 channel the values follow the actual (funcgen) values.