IMO, both Boeing and NASA are under quite a lot of pressure to not require further tests from Boeing because there are just enough Atlas Vs (the rocket that Starliner launches on) for Boeing to complete it's contractual obligations.
Any further tests would deplete that stock.
While I do think a certain amount of grace is due to a vehicle like Starliner - all space vehicles are relatively bespoke and have a consistent level of minor problems - its track record is not great. All 3 demonstration flights have had serious issues with the service module. And because Boeing can't examine the actual hardware once the capsule lands, they have a limited ability to determine the true root cause.
Case in point - NASA and Boeing throught they understood the helium leak before launching... and after 4 more leaks popped up realized that they actually didn't.