The percentages are not the percentages of total battery loss, they are the total amount of time active for the amount of loss you have had. As far as I understand it.
Ie: You have lost ~15% battery, but the total number of % listed is in the 50% mark. (+ All the system apps)
A new Android security update came out. Maybe it was installing it in the background?
Maybe you had low wifi signal and do your phone has to use more power (and generates heat) to connect.
There are many reasons why a phone can get hot. Can't trust those battery numbers unfortunately