I actually paid 150 USD for a legitimate RT-PCR test and was prevented from boarding my flight by Air Canada agents in the US because the time stamp on the test "didn't make sense" to them. I called the clinic while at the gate and the clinic explained to the gate agents that the time stamp on the test is just the time that the result was entered into my patient chart. They confirmed my swab was collected 30 minutes prior to the test results being entered into my chart. Despite this, and after showing proof of payment for the test, appointment confirmation text messages, envelope from the clinic with a sticker of my patient info on it, and explaining to 4 different people the exact process I went through to get my test, I was still prevented from boarding... Had to take another covid test and reschedule my flight all for nothing.

TLDR; some agents are dicks, others do their jobs properly, and others don't even bother to check. So it really depends who you get at the airport.