Spells that are set to ignite things do not ignite barrels if they are broken by the spell in question.

For example, if I have a spell that is set to Ignite things, and it does Physical damage, and explodes in a radius of 4 meters; if I place a barrel of oil or ooze within 4 meters, the spell will break the barrel, but it will not ignite the oil or poison that immediately pours out.

It will only ignite (bar other ignition sources) if I actually break the barrels first, and then use the spell in question.

I can only assume that this is a bug or oversight.