>Bugs can always be introduced into master.
Or you can cut a release branch, test, and iterate on that branch and be secure knowing only those release iterations are being made.
Its fine for master to have bugs if you're on a release branch. That's why people use them.
The question is where do you want to discover them? You either run tests against the feature branch before merging or you have to gate your master branch into a version on a test system before it is promoted to production.
CI/CD can be chaos but the Facebook philosophy is that we would rather you delivered 1000 features per day and be able to quickly fix anything you might break in the process rather than release 1 thing a day that is fully tested. Might not work for your system but it makes sense to them.