It happens to me too. Sometimes even more... But i think its a bug, because when it happens, after that, the battery stays on same position for hours.
Ex: Once it discharged 40% on TWRP but then it lasted for the rest of the day with only 60% with 6 hours on idle without consuming any battery, maybe TWRP causes bad reading from the phone to battery stats. And when it happens, sometimes when i reach 20% battery, just plug the charger and it goes right away to 40% or 50%. This things only happens when i use TWRP, in other days it behaves normal, so i assumed that TWRP messes up with the phone readings about battery.