Generate a bugreport, download the zip from the phone to a PC, open Battery Historian
https://bathist.ef.lc or
https://historian.ivkos.com/ on a PC browser and upload your bugreport, analyse the diagram, look for kernel wakelocks, userspace wakelocks, top apps, foreground apps.. etc.. Probably sounds difficult but trust me it is not.
Root the device
install
BetterBatteryStats and check the values after half a day (or at least a couple of hours).
install
GSam Battery Monitor and check the values after half a day (or at least a couple of hours).
install
WakeLock detector and check the values after half a day (or at least a couple of hours).
What kind of modifications did you make on the ROM? Any flashed mods or Magisk moduls in use?
When detach the charger.. is your CPU stuck at 1.6GHz? Is it enabling VDD restriction by itself?
Hope this helps..