Out of curiosity, what IS the pH of the soil and the water you are giving them?
I use rain water, and make sure it's not overly acidic. Adding fertilizer to that water WILL raise the pH somewhat, so it's a factor.
This doesn't look like pH lockout though... it looks like a temperature issue but I trust you when you say there haven't been swings in temps.
I'm following in the hopes to learn MOAR hahaha
Haven't tested this year, chemical fertilizers are acidic and tend to drop PH unless pre-buffered.
Last year was around 5.0 and added lime / oyster shell flour to whole garden area, and just oyster shell flour this year. I do not see any PH related issues.