Always pH adjust after you add fertilizer. There is no point to doing it before. Is your probe broken? Re-callibrate it. Sounds like equipment issues. RO should be ~7, but maybe when there are so few dissolved molecules, it doesn't read accurately. This should not be a problem after adding fertilizer, if that is the case. Also, save yourself some money. Just use some 5% white distilled vinegar (acetic acid).

You can’t PH test straight RO. There’s no PPMs in the water the PH tester has nothing to read so the values will just aimlessly drift. Like m0use said you need to add your nutrients and then adjust the PH. This is why I don’t mess with RO anymore. It’s more work than it’s worth and you can’t PH it when you’re doing just a straight water with no nutrients irrigation. However I’m lucky because my tap water is ultra clean by industry standards. I’m at 60-61 PPM. So if you have used your PH tester in straight RO water, I suggest you do a recalibration on your device. Again like m0use said already, using your PH meter in straight RO can cause damage to the device.