I have this great big sweet potato vine growing in my apartment and all summer it's been getting bigger and bigger-- I recently moved it to a bigger container with some mixmatch soil and it's been doing well but I've noticed over the last month a lot of it's leaves will randomly go yellow and fall off!

I'm not sure why this is, I want to replace it's soil with better draining potting soil soon but a cutting of the vine that I have in water is having the exact same issue! I looked into it and it's base is nice and sturdy-- does anyone have any gardening idea about what's making it die out so slowly??

(also these are ornamental sweet p's not the good kind you can eat)