Iphone battery runs down after IOS 17.3 update

My 2yr old Iphone SE updated to IOS 17.3 about a week ago and since then the battery life is horrible. I used to charge maybe every third morning but now I need to charge it twice a day even though it gets very little usage during the day (screen usage says anywhere from 3 min to 13 min usage per day!). And then there was last night; I turned it off about 11pm and noted the battery was about 72 or 74% and when I turned it back on this morning about 8am, it said the battery was at 43% - HUH(!!!) it wasn't even on and the battery drained by 1/3??? After checking the weather and quickly looking at the stock markets, i.e., about 3 minutes total, the battery level was at 37%. I used my pc to research this and went to settings/battery/battery health just like the half dozen articles said and turned off this and that app and looked what apps were running in the background and found 'settings' was the major user. After 2 minutes doing this, my battery level was now at 27%, so I switched to low power mode. An hour and a half later, after not even touching my phone or waking it up, while it's still on low power mode, the battery level is now 13%.

Can someone explain this to me - I mean losing 30% power while it's not even turned on??? And then losing 8-10% power per hour while it's in low power mode and not being used? I checked the battery health and it says 93% max capacity and I've got optimized battery charging turned on, also turned down the screen to maybe 25% brightness (did that last week when first noticed quick discharging), so something is running in the background but what?