I’ve heard that happens when there’s excess nitrogen. Nitrogen helps the green growth of plants, so I feel comfortable being more generous to herbs like basil and parsley.

If I’m guessing, you should water more to dilute the fertilizer you’ve already put in and/or not use too much nitrogen in the future.

Here’s the first casual link I saw talking about it: https://blog.drjimz.com/effects-high-nitrogen-tomato-plants/

Sorry, if I’m assuming too much and this doesn’t apply to you.