I got this lovely plant earlier this year, one I've really wanted in my collection. it was doing fine, but recently I noticed the new leaves on some vines are coming in fully green 😢

it sits in front of a north & west facing window. gets about 3-4 hours of sunligbt in the late afternoon/evening and bright light throughout the day.

Any reason why it would be losing its variegation? its not on all the vines, only about 4/5 of them. I pruned one back a bit, and all the new growth points are solid green. Another had a giant green leaf but the new growth is variegated. (Thankfully).

I’ve attached photos, it’s a cloudy day today so no sun, but ut gets good light.