One thing to consider is that I've heard/read from several sources that it is not a good idea to spray neem oil where it sits all day in bright sunlight. It is best to spray on a cloudy day or at the end of the day. For some reason the sunlight and oil can cause something similar to the plant being sunburned, like if you don't harden off new plants gradually that have been raised indoors with plant lights.
I doubt this is the reason, but something to consider when spraying with neem oil in the future.
I've had Kentucky Wonder beans yellow and die like this but I never could figure it out.
