Doctor office says its covered but insurance partially denied

i’m not from the U.S but i am in the U.S working legally, this is my first ever experience of doing an annual checkup at a clinic that i found in my unitedhealthcare website, when i walked in i asked the front desk, the nurse, and the dr if it will be covered, all 3 says yes, after my annual checkup, the dr sent me to a different building (labcorp) to do a blood work, in the frontdesk after checking in i asked the front desk again and the lady said yes, fast forward to 5 days later, i see 2 claims in the website, one for the checkup which was covered 100%, but the bloodwork was partially rejected, they charged $275 btw and so i have to pay half, so i called united and they said the dr was supposed to request for a prior authorization before the procedure, then i asked so what should i do, they said just tell the dr to submit it, i asked even after the procedure it will be okay? they said we will review it, so i called the dr office but another nurse picked up and i told her what united told me, i also emailed them explaining the same thing, the dr office replied saying i need to contact labcorp billing department, so thats what i did, called them and they said nope, the dr office must submit letter or email to united.

Is it normal or expected condition or behavior in the US where as a patient you will be thrown around to different office? Is it the responsibility of the patient to submit prior authorization before doing anything with a dr? Is there an ombudsman or a watchdog at a state or federal level that i can ask or report to?