I've seen this sight many times unfortunately...
Root rot or fungus.
Root rot is normally proceeded by constantly limp leaves. Edit: on more mature plants they do go brown on the ends too, and maybe a bit yellow. On younger plants they just go very very limp but tend to not go brown or yellow before dropping off.
Fungus is mainly normal leaves, (maybe just brown underneath), that just drop off. Most of my plants end up with one or the other.
Hell, even many of my new seedlings are doing this now
I recycled some potting mix that must have been infected and I'm getting fatalities every day even though the plants are 2-3 months old now.
Makes a change from the fruit flies, thrips, mites, white flies and aphids I guess.
The white flies or whatever they are is new.
I had some plants in the ground doing badly and didn't know why. By accident there was a thriving flat leaf parsley growing underneath one. One day all it's leaves were curled up, the weirdest twisted parsley plant I saw (and it was growing fine for months, a very quick transformation happened). Some flying bug inside I guess white flies. Wow, a new pest, hooray.
Meh, such is life. For every 1 plant you want, sow 8 seeds, raise 4 seedlings... Forget caring about them as individuals, too many pests, diseases, fungus and genetically poor seedlings possible.
Seeds are cheaper than time, soil, pesticides/anti bacteria/fungus treatments to fix problems. If you are growing thousands then there's a ROI, so worth mass treatments. Growing a few plants for personal use, easier to cop the loss and learn/move on/have a backup plant etc