I wanted to edit my reply but now you can only edit within 15 minutes posting. I was going to correct the issue or delete the comment about trouble saving login information--I see you once you actually sign in somewhere it gives you the option to save it, even if it did delete all my Keychain info from iCloud. There workalso numerous dictation typos, so I'm reposting my comment here, corrected. By the way, one other thing I'm trying now is turning off Notify me when my name is mentioned, in the message app settings. I don't know if it will have an effect, but perhaps there's some connection to Siri in the background. Now my corrected text:
I have had this problem periodically in the past on my iPhone 12 pro, even after I disabled every possible Siri function in every app. Then it tended to go away until somewhere around IOS update 15.3, when it was periodically using various low and high percentages, including 100 percent. On someone's suggestion I signed into iCloud, which I had never set up, to see what it was doing there, although it seems just signing in turns things on automatically. That's when I discover that starting February 2, at least that's the day that of the first photo, my iPhone started uploading my new photos to iCloud, even though I never turned it on. I deleted them from my iPhone iCloud after some worry about it deleting them on my iPhone, but I already have the full resolution versions on my iPhone. One has to go through numerous procedures to turn things off. However this seemed to solve the Siri problem at first, but then it came back. I went into iCloud again, and it turned things back on by default, including a couple other things that weren't on before, but kept a couple of things off that I turned off. I thought maybe I'll just turn off everything and not sign out, and tried that for a short while, but I wasn't happy with that. So I signed out and it asked me if I wanted to download my Keychain and password data to my iPhone or else it would be deleted. Perhaps I should have done that but I didn't and then call my login information was gone from my iPhone. I signed in again to iCloud trying to find out if I could restore the password info, but I could not find it in iCloud. I saw information on Apple's website that, even though the pop-up warning said that the password info would remain in iCloud, the Apple info said if you delete it from the last device they will be deleted from iCloud servers also--this is contradictory and confusing information, especially since iCloud delete it from my device, not me. I never signed in to iCloud with my MacBook and it does not seem to be connected to iCloud. I think this is abhorrent of Apple to set up iCloud regarding passwords and more recently for photos, after the IOS 15.3 update, I believe, without me choosing to do so or even signing in to iCloud--this is bad behavior, and has caused a lot of problems. I also noticed after the signing in and out of iCloud that it turned on iMessage on my iPhone which I had never turned on, or if it was on, I turned it off at the beginning--I got my first iMessage, which is how I noticed it was on--I turned it off again. Getting back to the Siri issue. After seeming to drop down to an increasingly low percentage, to 1%, I just noticed after 39.5 hours since full charge, not using my phone much, if at all, battery was down to 85%, and it showed 100% usage by Siri. Even after I additionally played some videos in the VLC app and battery went down another 5%, it still showed 100% Siri. However after I plugged it in to charge and checked the battery usage, Siri had completely disappeared from the list of usages