As somebody who lives on Dartmouth Rd right by the junction in question I don't think it has much to do with the phasing, though they might have changed a little, it's more to do with the traffic backing up on London Rd down to the junction and drivers not keeping it clear for when the lights aren't in their favour.

It usually happens when there's roadworks further up the South Circ.

I can tell when it gets bad because I hear right outside my window someone honking their car horn continuously at somebody stuck in the junction as if they blow their horn long enough the traffic will magically disappear.