You have to keep in mind that the way of displaying those percentages have also changed. Mine was also a long time at near 100% every time and then sudden drop. Because probably iphone was taking the mean/median value of the last 6 month observations or so.
Right now, it is still lagged version but not too far away.
In the dynamics, i would have more trust in coconut battery readings history rather than comparison of battery health percentages in different ios versions among different Iphone models.
If you were to have Coconut Battery history for all of them, then the outcome would be more precise, since the coconut takes the current/immediate readings, while ios battery health is just averaged number based on n amount of previous month readings.