I noticed this happening on a few plants late last summer.
There are many reasons this could happen. One is too much Round up. Another is too much phosphorus. The other is too little of magnesium or iron or zinc or nitrogen some combination of them.
If this is a magnesium deficiency then Epsom salts ( magnesium sulfate ) are needed. Add 1 or 2 teaspoons of Epsom salts to 1 gallon of water. You want to do this 3 times at 6 week intervals. Older leaves are most effected by magnesium deficiencies.
If this is an iron deficiency new leaves are most likely to show the symptoms. Adding organic material helps with iron deficiency or any other treatment that breaks up the soil and lets more air get in. Adding peat moss and or 1 pound of sulphur per 100 sq feet of garden will also help by lowering the soil pH slightly.
For zinc you want to add fertilizers for acid loving plants. Too much phosphorus or alkaline soil can cause problems with plants getting enough zinc. Often leaves will also be smaller and or curled.
Nitrogen deficiencies show in older and lower leaves first.
If your soil is alkaline, pH > 7.0 as most clay soils in the area are, you’ll do better to apply zinc, magnesium and iron as liquid fertilizer sprayed on the leaves. Alkaline soils tie up the nutrients faster than the plants can take them up.
The best thing you can do is a Soil test then you know for sure what the problem is and what you need to do to fix things.
I find some of my plants do this when they require nitrogen and that tells me it’s time to fertilize. The plants in my garden that show the need for fertilizers first are: Mexican Bleeding heart, Passion vine, Gardenia.
Micro nutrients needed by plants