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.

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