Again... People seem to be making their own rules for a stitation that the DOT already calls out

>Generally, no. If you have met the following conditions, airlines are not allowed to deny you permission to board, or remove you from the flight if you have already boarded the flight: You have checked-in for your flight before the check-in deadline set by the airlines; and A gate agent has accepted your paper boarding pass or electronically scanned your boarding pass and let you know that you may proceed to board

https://www.transportation.gov/individuals/aviation-consumer...