The battery drain is definitely associated with wifi. This snapshot of the Battery settings shows (from midway across) discharge with wifi off then, after lunch, the wifi on. The graph is much steeper with wifi on.
The start of the graph is also with wifi on, then it moves to charging at night.
My guess is that 14.xx introduced some background activity whenever connected to wifi. Maybe artwork for music or similar unwanted downloads is active? I don’t use iCloud for photos or backups so it can’t be that. I do have an Apple TV and Homepods so handshaking with them is a possibility.
I feel like giving Feedback to Apple suggesting that they look at this discussion!