From my experiments this morning with the new 5 pole sine filter I've discovered that there is some degree of load sensitivity on the output. I've seen this in the past when disconnecting loads - sometimes the mode will drop one notch while the PLL compensates. But somewhat more disturbing is that if you put a 50 Ω load on both outputs at the same time, you'll see a small glitch in the sine wave at the rising edge of the square wave (the two aren't perfectly in phase because the sine filter adds some delay).

Fixing this means either lowering the power supply impedance to the output buffer chip or adding more bypass capacitance for it. I'll have to play with it some more, but for now my best advice is  to use one output or the other.