Have you check in the east in Yi Ti to see if winter is still there?
If it isnt, could you please upload a save from when they were around?
They never spawned. I also checked Yi Ti, but they weren´t there. Unfortunately, I don´t have that save, because I switched to Thenn, manually launched their invasion, saved it again and returned to my original character.
Even then, when they took Thenn, it became independent under some ruin, with WW standing with their event spawned army in Shivering Forrest. So I used console to give them Thenn and SF, which fixed it and they started attacking Wildlings and short time after that got beaten in Battle for the Dawn (although they had that derpy bug for a short time, when bigger army stands in its land, while smaller army sieges its provinces and it never moves, I once had it with Volantis and their liberation revolt, where Volantis army, superior in numbers, stood idle while rebels sieged their counties). Then summer came and since then, summers and winters were working perfectly.
I don´t know if there are any events regarding that long winter with WW in Clash of Kings, but it was first winter after start of that game, so basically the one in Feast for Crows scenario, which also doesn´t end until War for the Dawn is won.