Why is my ph dropping so fast?

the plant looks fairly healthy so I would first calibrate the ph metre to be certain of the actual ph of the soil

are the plants healthy? or is this ocd? need to clear that up first.. and more info needed. runoff is only slightly representative of what the pH is inside the substrate.. you in no small part flush out a lot of what was in there from previous fertilizations in any runoff, so that will be different just from that. you first need to learn a baseline of what the runoff reads when things are going smoothly over a longer period of time.. now, you can campare to whatever value that is and recognize an issue before it causes anything major or help diagnose other issues above ground etc... you are measuring dirty ass water that has sat there for 1-3 days or more. evaporation has occured. If it is soil nutes, some stuff does not enter plant but stays in substrate until microbes break it down.. that creates a situation where the EC is higher in the substrate than what enters the plant -- what enters the plant is what matters. That buildup for th emicrobes to chow down on and poop out the bits that can fit through the roots can also impact pH, but again, the parts taht cannot enter cannot impact the pH inside the plant which is where it matters more. So, if it is healthy and remains that way, all is well... and you've learned what "normal" looks liek when measuring runoff... volatile and anything but what the actual substrate is.. but still mildly helpful.

forgot to add, if you think its a meter issue, you can calibrate the meter to confirm its good, This should be happening on a monthly basis or at min, the start of the grow and the switch to flower. Some people do this monthly, old meters will need more love then new ones. Some have an auto reminder to calibrate, most do it. Storage solution is also important and can effect the reading. If my pen dried out I let it soak in the storage solution a good 10min before using it. and if it first time using it in months I let it soak for a day or two as if I don't the readings will be way off then I calibrate it. I scrub the PH bulb with vinegar water once a year to get off any buildup of scum on it with an old tooth brush. gentle pressure and nothing abrasive like soda ash or toothpaste.

Could be a meter issue or possible buildup in the soil but if you’re watering everytiwith like 10-20% runoff I wouldn’t think it would be that

runoff is normally going to be a bit different then going in. but it should normalize out by the end of it all. eg 2L runoff the last 1L or so should be stable. If you are collecting the first bit of runoff and measuring it, don't. Wait for over 50% to run out, then only collect the stuff after that and test its PH and EC if needed If you are growing in soil I don't normally get runoff with it. but soilless I do if using liquid nutrients. Sometimes shit can get into the medium and grow their. bacteria or fungi and it can alter the PH of the medium. This is one cause of why PH may drop and its harder to solve as you need other bacteria to out complete it. Or you just need more runoff. aorund 10-20% the pots volume. I've also noticed any time I grow in only peat moss aka promix my PH is always dropping a lot. for me it need extra buffering with calcium or rock dusts. You can try using PH up like potassium hydroxide but untill the main issues is corrected its only a bandage solution.