Could be anything from a water issue to a nutrient issues to a pest or disease issue.

Pests you should be able to see. Watering on a fixed schedule can result in too much, too little, too shallow, etc. especially with drip systems. Length of run time and size of emitter is what counts with them.

Leaf color is quite dark in the photo like maybe fertilizer burn but that could just be the photo. There are common bean diseases like rust, Early Blight, and Alternaria but it really doesn't look like either of those from the photo. You can Google photos of them for comparison. Either way, remove the damaged leaves.

It could just be heat stress and/or sun scorch. Are they producing? How old are the plants?

Dave

If it’s affecting (especially only) the upper or outermost leaves, I’d think sunscald otherwise I’d vote pest …though I couldn't tell you, with any certainty, which. A week or 2 ago, I had something similar on a Kentucky Wonder (or Cherokee Wax?) bean. Only two or three leaves on one plant, the rest on the plant and those on neighboring beans plants unaffected. I removed the leaves, continued to take care of them as usual. It’s now nice, full, healthy & productive.

Thought of another possibility - have you sprayed them with anything? Anything at all? Like Action Claw said, beans are pretty resilient so a few bad leaves here and there isn't a big deal in the long term.
Dave