blessed lamps (planted on reinf floor) burn out in buildings in caves

[Ekcin 2,407]

I recognized that phenomenon lately (I have a priest only since short time therefore not so much experience with blessed stuff), and I observed it only in caves off deed. Normally, a blessed lamp planted on paved or reinforced ground will stay lit once filled and lit. But with lamps inside buildings, that is not the case. They are burning empty much slower than unblessed ones (which run out of fuel in 3 days or so), and seem to get unlit irregularly, if I observed correctly. Afterwards, they can simply be refilled and re-lit. It is a bit unpleasant though.