Yea based on everything on thid topic that sounds like the most likely cause.
@op As for watering amount. I give about 1.5L or bit less per plant in 11L pots. There should be a proper drainage for excess water in the bottom of soil and you should get a bit of run off when you water. But giving a lot of water at once to dry soil will make a lot of it run off, so give it first a little bit of water, let it sink, give it some more, let it sink and then give it rest of the intented water. I water when the plants seem to want it, or when at least one of the ones that drink about the same amount shows signs. Sign is that the leafs start to droop a bit, but dont let them get too droopy. I also lift the pots to make sure they are light(=dry) enough and sometimes stick my finger in the soil to see how dry it is. If the soil is even a bit moist at like 1.5 inches deep and the plant droops, the drooping isnt because of dryness. As you have noticed overwatering also causes drooping, drooping because of w(nting water looks a bit different than drooping because of overwatering.
Think about the whole watering like this; roots need oxygen as the upper parts of plants need co2. If the soil is too moist, oxygen doesent get in properly and roots cant breathe. Giving a bit too much water every now and then wont cause issues, but overwatering constantly is comperable to not bubbling(=oxygenating) the water in DWC. Its better to let the plant dry up bit too much between watering than to keep it constantly moist.
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i been followin this, and tbh, can't really figger it, lol 
i too uze an old style probe moisture meter, stuck all the way down to the bottom when i check it...when the bottom is in the red, i water, but not before...sumtimez i'll even go an xtra day...remember, it'z a weed & can handle the dryness jus fine 
it duz seem that 1/2 gal ea twice a week is a bit much tho...also remember the old adage that there'z no such thing as overwatering, but there is such a thing as watering too often 
ppp
