Sorry my friend but I think this is a bug!
Yes, it keeps an unusually long time at 100% but after that the drain is to fast (compared with time @100%)
In my opinion, it needs while charging between 99% to Full unusually long.
Meanwhile, me reminds this batt to a car tank.
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I do not think it's a bug - it took me 10 minutes to fully charge my phone from the state above (via USB!). I also get 5% drain in 24 hours with phone mainly in Idle - so maybe it stays a bit to long on 100% but not that significantly as you say!