It is very common for the pH to rise overnight and then start retreating. Once the water is an established tank it might make a drastic change. The Malawi do not need to be in a particular pH inasmuch as they need it to be stable. I do not worry abbout pH as much as I watch my other numbers. By the time my tank had cycled, the pH dropped to 6.4-6.8 and stays there. I do not adjust it at any time. Almost all my tanks generate the same reading. Setting my water out overnight gets the same numbers as yours but once in the aquarium all bets are off.
FWITW - I have a shallow well.
FWITW - I have a shallow well.