I found this reverse engineered answer of how team power (CP he called it) is calculated. Kerridoc doesn’t claim it to be 100% accurate, but it looks close.
I would suggest that in order to fix this missmatch which badly over-ranks partially leveled or unleveled heroes with higher stars, the calculation could be tweaked, perhaps like this:
The component which is 40.26 * stars could become 40.26 * stars * (current ascension tier) / (total ascension tiers). That way a 4* or 5* hero would be ~30 points lower at tier 1, ~20 points lower at tier 2, and ~10 points lower at tier 3.
I’m guessing that most 5* heroes not yet maxed will still hit a bit under their team power with this system, but not as badly.
No idea what the impact there would be on non-maxed 3* heroes or 1* and 2* heroes, but I seldom pay attention to less than maxed 3* heroes or any 1* / 2* heroes anyways; they are all food, not heroes I use. 