If battery is 3x lower, from an update you sould contact EU consumer protection.
This is planned obsolescence, legal speaking. An update shouldn't reduce the capabilities of our phone.
Im still on Nougat and im getting 4,5 screen on time (with full wifi &calls).
People from Oreo and Pie reporting much less (check the entire https://eu.community.samsung.com/t5/Other-Smartphones/S8-oreo-battery-drain/td-p/476750).
Now i have red tint on sreen after 1 year of use without any problem and any update.... Samsung is trying to make me update to the new OS....
We should all contact EU consumer protection. Such matters are sovle only with publicity!!!