I have good signal here, i tried to put sim card in other phone (Xiaomi MI A2 lite) and there is no drain with 4G. Since i work from home, phone is connected to WIFI and data. Most of the time it is lying on my desk. And i'm talking about S21 exynos version. First image is my battery life with 3G and second is with 4G.
I take it back, strange that the drain would happen whilst on WiFi, as usually this turns off mobile data unless you chose to keep it on. Definitely worth reporting to Samsung.
As a potential short term fix, set up a Bixby routine to switch to 2g when you're connected to home WiFi. Appreciate its not perfect but should help till Samsung fix