It looks like an environmental stress of some kind. Candidates include cold, drought, sunburn, wind burn, etc. Often that is the result of planting out greenhouse or polytunnel grown seedlings before they are properly hardened off.

Thanks for replying. I'm new to cucumbers. They're still inside the house for the moment - sitting on a sunny window sill.

Hmm..that's strange! Well, then, the next questions are, have they recently been moved to different window with more sun? Have they been placed between the window and the curtains over a cold night or two? Were they left outside to "catch some rays", and then forgotten? Were they left in the care of others, and possibly allowed to wilt badly? Were they recently transplanted, and what was the medium used? Are they growing within the range of possible furniture polish overspray? Were they removed a full safe distance while the glass was cleaned?

Welcome to GoY
due to the extremely hot weather I suspect this is sun scorch. have the leaves been in contact with the glass? if yes then they have just got too hot/sunburnt.

are you intending to plant them outside? if yes then put them outside during the day and bring them back on a night for about a week and then plant them out.

I'm no expert but when I first starts growing cues in a greenhouse, I lost complete plants due to red spider mites even though I had damped the floor each day with water. I lost the first two plants and had to buy new ones and sent off for a natural predator from an online company and that worked.