Yo! Air leakage can happen due to manufacturing defects in the material or the gaps in the externally connected parts of the dome. It can also happen due to operational errors (like the vent not working as intended).
The system is not made airtight because the aim to test how such an environment can be controlled due to dynamic changes in the system. Such a dynamic environment would be similar to how it will be if we were to build human habitable modules for real.