I posted my stand by drain getting worse suddenly in other thread. It used to drain only 3~4 overnight on WiFi. But from couples weeks ago, it drained like 20~30% doing nothing overnight. I tried FC, cache wipe with little to no effect. But I found turning off WiFi stopped excessive stand by drain. I think others with SoT less than 3hrs probably are having bad stand by drain too.
I'm on at&t and I read that some others on this carrier had the similar issue with S6, S7. But my wife's J3 (2017 model with Exynos 7570) seems to be amazing in stand by drain compared to mine, that one has only 2600 mAh battery. I think Exynos being superior to Snapdragon in battery is probably true. But this is probably bad OS update issue with at&t.