Sounds like you may have a warranty issue. I upgraded my 5s and there was a sudden and rapid drop in battery levels immediately afterward. I would have blamed the OS, except my Touch ID also stopped working. I took it to Apple and they blamed the update exposed some hardware defect, but the phone was out of warranty so they would only recommend sending it in for repair for $250. And while they could see there was a definite battery issue, they tested the 2-year old battery and declined to replace it as it tested within specs ... this was obviously prior to the throttling issue after which no questions were asked. Two years later, the battery swelled and Apple replaced the whole phone for the cost of a new battery. A presumably refurbished 5s running iOS 12 has better battery life now than it did new, albeit running somewhat slower.
Of course I’m speaking to my issues with a 5s and SE, never having used a 6s or 7. Yours is also anecdotal, but perhaps the 6s and 7 are more susceptible to iOS updates? Or, there are defects being exposed with the upgrades as in the case of my 5s? I presume you’ve factory restored, and reinstalled the phones being upgraded after experiencing these issues, and the issues persist? I find sometimes a clean reinstall solves some persistent issues as well. What has Apple officially told you about these discrepancies in power use following each update?
The devices weren't mine, they were friends' devices. No hardware defects resembling yours occurred at any point.
I have had regular contact with those devices and I have specifically checked battery life myself on several occasions.
There was an immediate drop in every iOS version with no alterations in usage, with every device I have seen and tried.
Regarding defects, the devices work well, only the battery life immediately tanks. A few slight quirks might show up here and there; an odd crash, some slight hiccups, but nothing significant, in my experience; not with newer devices (32-bit devices are a different experience. iOS Updates shredded the device into near uselessness. Performance was pathetic and battery life was abhorrent. Apple seems to have the performance somewhat straightened out - they seem to work better than they used to - but battery life is still the greatest issue. With this, I do not mean that updates don't bring about performance issues, don't get me wrong; but those issues are less severe than those encountered with 32-bit devices, which were brought to a standstill).
I don't know if they were clean installed, I don't think so, but I have read that whatever improvement in battery life that might occur, it is minimal and under no circumstances does it manage to scrape the surface of initial iOS versions. That might be the solution for a small improvement, but I don't think it'll get to be what it was on the original version.
Still, that cannot explain why an iPad with 23% wear showed no significant difference in battery life when it wasn't updated; why my devices don't show any difference at all when they have 8-10% wear, but they remain on original versions, while an iPhone 7 with similar wear than my 6s - which gets the same battery life it did when new - had around 33% less battery life, and the only difference was that it was updated, while mine was not; a 6+ dropped from 10-12 hours of battery life on iOS 8 to around 7-8 hours (less than my standard 6s on the original iOS versions) only because it was updated. That 6+ had around 80% health, but my iPad showed no significant difference, with 77% health.
Example after example; test after test. The only correlation is the iOS version.
I use an iPhone 5c on iOS 9 (got it from a family member, it wasn't new, hence why it was updated), with 87% health, and its battery is poor. It gets around 3-4 hours with LTE and high brightness. My 5s on iOS 8 (widely regarded to have similar or even slightly lower battery life than the 5c), got around 6 with that usage.
I am absolutely sure (although I can't prove it), that my 5c would get 6 hours if it were on iOS 7.
Another example: I tested my iPod Touch 5G on iOS 6.0 when I got it, on December 2012: 6 hours 16 minutes of on-screen usage, mostly web browsing.
A second test was performed on February 2015, same usage: 6 hours 30 minutes.
A third test, June 2018, same usage: 6 hours, 36 minutes.
6 years later, with the same battery life it had when new, only because it is on its original version.
PS: Apologies. This was far too long.
