Have you turned up the screen brightness, or started using it in a brighter environment? Has your access to WiFi or mobile signals changed? You haven’t mentioned that you’ve started using OneNote a lot more intensively. Could that not be the explanation? Some apps use more power than others.
One Note and Facebook are notoriously high battery killers.
I've always used OneNote extensively and Facebook quite often without any issues. In fact, if you look at my first screenshot, I used OneNote a lot that day and the battery usage was only 50% with 6-7 hours of active use. My screen brightness is always somewhere at 25%.
My usage patterns have been very similar which is why I find the battery drop so jarring; it simply didn't last as long between charges even though I was using it similarly compared to last week.
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Last Monday's usage - almost six hours of usage (almost 4 from OneNote) and 60% of battery used. Pretty normal.
Last Tuesday the battery life dropped off a cliff for some reason. Didn't think much about it since I thought I was using it more than usual.
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Last Wednesday: back to normal, roughly 40% for 4 hours of usage.
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Last Thursday: battery just became poor again and never recovered. Has been barely 5 to 6 hours on a full charge.
Brightness is always 25% at most because I’m usually indoors and I don't like bright screens. As I finished typing this, my battery is now 79%. The first screenshot was taken 9 minutes ago. That's barely 3 minutes of usage per % doing lightweight non-gaming tasks.
The worst thing is that this is also happening to my relatively new iPhone 11 Pro which has never gotten past 5.5 hours of screen usage on a single charge since I got it even though my friends and most people posting their battery stats are getting 8-9 hours. Apple genius has said there’s nothing wrong hardware wise with my iPhone, and despite restoring as new the iPhone still experiences subpar battery life, and now my iPad is affected as well.
I’m suspecting it's an iCloud thing, but the battery still drains on my iPad when iCloud is turned off.
I’m going to try restoring my iPad as new, and not sign in my iCloud at all to see if that fixes the issue. In the meanwhile I’m really hoping someone with similar frustrations could shed light on whether they managed to fix this.