Enclosed Lava still sets nearby objects on fire?

Offline Mode, Survival.

A completely enclosed a vertical column of lava (stone and sandstone on 4 faces, Stained Glass on 2 faces), set wood objects on fire a few blocks away from the column. I can understand if there was any part of the lava exposed, but this was completely sealed/contained, and the seal was not breached. If it set nearby objects on fire, it should have burned through whatever blocks were sealing it yes?