To my understanding, the situation is that the 'service module' to the capsule is what's having helium leaks, but the capsule has redundant thrusters and can deorbit on it's own. Because the service module can't reenter the atmosphere, there's no way to look at it after the astronauts return, so engineers' only opportunity to analyze the issue is before the Starliner returns.
Not a safety issue, but a capability issue. And the ISS has the supplies to support the astronauts for a while, so why not. A quick up-and-down was probably originally planned as it reduces risk in some respects (what if there's unknown capsule problems?), provides more flight data faster, but now you get more crew flight time and the ground control can investigate known problems.
