Yes the longer coffee sits the stronger the coffee gets.
I notice that in my coffee pot that as long as the coffee pot is on and the coffee is being heated constantly the coffee gets stronger the longer it sits.
The coffee also gets stronger even when the coffee pot is off and the coffee is cold.
Sometimes I will drink a whole pot of coffee in a day and sometimes there may be a bit of coffee left in the pot that sits out overnight.
Then the next morning it gets stronger.
But I just add some water to dilute the coffee and then heat the coffee up in the microwave and it's just fine.