Hi,
I have been experiencing the same problems this week: battery drain that seems to be caused by Wifi even if it is supposed to be off (Wifi scanning was also off), phone hot on the back. I have seen this at least 4 days in a row. No big changes lately (update applied in July). The only thing I could think of was some new app. I suspect Chrome: I am on Fairphone Open and uses Lightning but I had to install Chrome to set up an app and I forgot to uninstall it right away after the set up.
Anyway, I uninstalled the 2-3 apps I had installed lately, I cleared the cache of all apps (via settings/storage), I made a battery reset (when my battery was empty) according to these instructions:
Today, the phone is not hot, no wifi shown in battery usage and it appears to be no battery drain.
I don’t know if it can help someone, as everything I did had already been advised on the forum, but I just wanted to give a positive feedback!
I don’t know if Chrome was responsible, but I like to think that to comfort me in using Fairphone Open OS 