Hmmm...I've seen this now and then on some passionflowers and have always thought it was probably iron chlorosis, so adding some iron and acidifier sounds like a good strategy. Some of the cold-hardy passionflowers stress here a bit in summer, but most types do really well. Hopefully someone else will chime in. I'd do a couple of light fertilizations with miracid or another acidifier and see if the new foliage comes in greener. Keep us posted, and good luck!

Mine were kind of yellow when they got here in the mail. They really greened up with some fish/seaweed emulsion.