[diedyesterday]

Of course the rate of expansion is not constant; That's the whole deal with dark energy which is understood to drive accelerated expansion of space due to its negative pressure (when you make something with negative pressure larger in volume you have to do positive work on it or give it energy and that energy is dark energy).

But the Hubble tension is something else: It's about measurements of H0 (the current value of the Hubble parameter) using two different methods (early universe and CMB OR recent universe and supernovae, etc.) that do not agree. The disagreement is about two estimations of the current rate of expansion called H0; Everyone agrees H(t) has been increasing (=accelerating expansion); But accelerating toward which recession velocity is the question.