Why is this any different then a fridge. And if the issue is bishul why does it make a difference if the fire is on or not?
A fridge would only be an
issur derabonon if the compressor turns on the issue with an oven is Mavir lighting a fire not Bishul. If food is not fully cooked then you are not allowed to close oven door even if the fire stayed on.
Fridges that have a heating element for the defrost need to be on Shabbos mode as it takes away any chashash of isuur deoreisa
(Defrost cycle should not be dependent upon the number of times you open the door and how long you keep it open.)