Turns out I do have both the In and Out S/PDIF cables connected to my AI (18i8) here. I'm not using the DAW out/KPA in channel, just have the cables plugged in to both channels.
At the friends house (6i6), I was connected to KPA Out only.
I'm not using the re-amp feature at all, but yes, with both S/PDIF cables connected, Internal sync works fine for me, at the moment.