A poor fit, minor damage, or any defect allows convection within the fiberglass. R-value assumes zero air movement, which is why it is far less useful comparing fiberglass to foam or aerated cement. In real world settings, aerated cement will outperform fiberglass.
The same thing can happen when concrete cracks, which is quite common. All of this a question of basic building maintenance rather than the building materials chosen.
Cracking is why single family homes generally avoid concrete walls in the permafrost.
I've never had a draft through a fibercement/wood/fiberglass/drywall wall, in any case.