So I did the line swap and the beer poured fine out of another tap, so I took apart the affected tap, the QD and bled the line again, cleaned with hot water, everything and put it back together again and wouldn't ya know, same spitting with co2 bubbles coming into the line again after reassembly. Weird.
The only thing I can think is perhaps the old regulator I used is off a bit and the beer is in fact over carbonated even though I had it set at the same 10psi. It's not that bad so I don't think I'm going to go through the hassle of degassing it and starting over.
Any other thoughts I might be missing?