Can you link me your ph up and down?
The problem is tap water comes out high in carbonic acid because it's a sealed system under pressure and all CO2 created is forced in the water and forms carbonic acid. So when left out the ph will rise as the amount of CO2 tries to reach equalibrium with the air surrounding it. Once it does the ph will stabilize but that does not mean it's stable enough for our need as that will be determined by the buffering capacity.
The time it takes to become stable will be determined by the amount of exposure to air, mainly the surface area and agitation. It's good practice to take tap water and aerate it for a few hours before phing or adding nutrients. This will ensure that the CO2 in the water and air have reach equalibrium.
If you are putting it in a jug the exposure to air is low and it can take a long time for this to happen.
So here is what I suggest.
Take tap water and aerate for 2 hrs then add nutrients and ph down to the desired ph. Then if in a jug seal cap it and it should stay fairly stable. When you aerate water the ph will usually rise and this is because of the of gassing of CO2 so by aeration first you are now ph adjusting water that has been aerated and therefore when you expose it to air or further aeration it will not change any further.
There is a bit more to this all but if you can link me the ph up and down I can check for other factors that may be contributing.
Are you in soil, hydro or soiless? There are different practices that are ideal depending on which.
Also are you using any products containing silica?
What is the order that you are mixing any and all nutrients, ph adjusters or supplements?