It could be several things so I think you might need to try different things and rule them out one by one. Check the soil and see if it feels wet. They are quite prone to overwatering so you should let the soil dry out between waterings and make sure it doesn't sit in wet soil. 😊 Also is it in a large pot? They have deep roots so need a pot twice the size of the pot it came in originally to let it grow out and establish. ☺
@ublaszko to be honest I rarely water it, only in summer when it feels dry. I had one before and killed it by overwatering 😂 so I leave it alone now. It’s in a good sized pot, well drained and never standing in water. I read frost can damage the leaves this way but we haven’t hardly had any here so not sure that’s the issue either 😜
It doesn't look like frost damage and the temps would have to be -5 for a prolonged period. It's not leaf chlorosis either as it isn't showing darker leaf veins. I think it looks like nitrogen deficiency because it seems to have lighter centre similar to my hellebore earlier this year. I don't know why it would have nitrogen deficiency though. Can you see any white build up on the inside or outside of the pot?
If there's white build up you'll need to rub it off and flush the soil with plenty of warm water and let it dry out before doing anything else with it. Otherwise, I'd use slow-release general purpose feed but wait until it finishes flowering and shows signs of new growth. ☺ I've had some experience with nitrogen deficiency but it could also be something unrelated. It shouldn't hurt though to give it a small dose of slow release food when it's actively growing.
I agree with all of what @ublaszko said, I would like to add that I have found PH issue with pot grown and even some times soil grown daphne generally in alkaline condition showing similar leaf conditions nothing that tablespoon of Epsom salts won’t fix watered in.
Nice 👌 My Daphne (sounds like an elderly aunt!) looks to be at the same stage. I must take a fresh look at it soon & not miss it flowering as I normally do. Meant to be very scented too, but always missed that too 🤨