Hi,
I found this kind of "Bug" here and there over the Years of playing DayZ.
I did a full Video about the Topic for you to take a look on the subject.
Short: Firebarrels /Fireplaces dont produce light inside a building. The light will render in if i look into the building from far away OR if i stand in front of the lightsorce but looking in another direction.
I guess this Problem takes place a lot, if the Fire is placed inside a building on a higher ground.
See the Video here: