What might be causing PH to drop in the rootzone?

Plants look healthy besides some minor cupping close to lights. They are almost 4 weeks into flowering.

I've been doing the 1:1.5 extraction analysis for the rootzone 2 times a week and I can't understand why the PH level drops to between 5.1 and 5.5. Measurements are taken before watering and I'm watering every 2 days (1 off 1 on 1 off 1 on...). The thing is that PH of water that goes in with nutrients is 5.8-5.9 and I have always heard that in coco with healthy plants ph swings up in time, not down.

In week 2 readings were 170-280ppm in the rootzone with 1300ppm going in. I've been slightly increasing the ppm of nutrients with every watering and now I'm at 1500ppm going in and rootzone at 450-470ppm. Canna says that target value for EC in the rootzone should be 1.1-1.3 which is 550-650ppm. I'm still not there yet so sounds like a slight deficiency. Trying to dial in with my new sealed co2 environment.

I have heard that too high ppm can lower PH, but I don't think that's the case.
Another thing I've heard it that bad bacterial growth in the rootzone can lower ph, but that too doesn't seem to be the case, because I have 2 different strains in the room and one is 2 times bigger than the other, which means that the bigger ones drink water faster and it's highly unlikely that they are the ones being more bacterial than the smaller ones. And here's that: I have now increased the ph of nutrients going in from 5.9 to 6.4, but still the same drop and the bigger ones show me lower ph than the smaller ones - my last readings were 5.5 vs 5.1 and one week ago it was 5.3 vs 5.5. Do you get it?

What could be the reason?

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First of, I've never done coco, so you may want to wait for experienced answers. But in both dwc and aero/nft, higher ECs equaled low pH and rising PH meant they weren't eating fast enough. I'd drop back to around 1000-1200ishppm and feed at 6.2-6.4+ph a few times to try and get the coco pH up.

Exudates are the roots leaking acids around the root zone and this will drop the pH in an attempt to attract microbes that break down nutrients as well as dissolve and break down on their own. The rhizosphere will typically be slightly lower pH than the rest of the soil because of these exudates that the roots are giving off. It is hard to accurately measure however.