I'm having EXACTLY the same problem on both my phone and my wife's.
Battery life used to be absolutely fine - lasting a full day no problem. As soon as I updated (OneUI 5.1), battery life has been absolutely destroyed - down to about 20% by midday. Literally nothing else has changed, and it's the same on both phones.
I do hope this wasn't an intentional feature by Samsung to cripple existing devices and force people to buy new ones. It's not a battery issue, it's very clearly a software update issue - no other additional apps installed and the change in battery life happened literally overnight at exactly the same time as the update. Is there any way to roll back?