The expansion tank provides a relief function from too high a pressure in the cooling system and is therefore not a completely sealed system:
https://www.bimmerworld.com/About-Us...g-System-Tech/
"BMW Expansion Tank
The expansion tank is also known as the overflow tank or coolant reservoir. As cooling demands change the level of coolant in this tank will go up or down. It's also a place where coolant can spill into when system pressure is too high. The cap on the tank serves a vital purpose of venting/controlling system pressure. Too much pressure needs to be vented to prevent a failure. Too little pressure leads to poor system performance. For this reason the expansion tank, cap, and bleed valve are the highest points of the cooling system.
The expansion tank on 1992+ models seems to be the most common source of leaks and broken plastic. It must be a material issue or quality control problem that allows the plastic to split or warp and cause leaks. Failures are not an epidemic and they typically last 5-6 years so maybe that's just their expected demise. Unless the car is tracked or raced, most people simply reinstall an OEM plastic tank with the expectation that it will fail and need replacement again in the future. For ultimate reliability, but higher cost, we offer an aluminum expansion tank for some models."