Did the cashier give off body language suggesting they closed the line due to not liking the guy behind you?
I don’t know under what circumstances a cashier might open a line only briefly, but I often realize I forgot something right when I start an activity.
But yeah, if they caught a glimpse of the guy behind you and then closed down in a pissy way, I’d probably start by offering to be a supporting voice during a complaint to the store management. If it’s any kind of pattern, the cashier will likely be out of a job.
Bias can be subtle and take work to suss out, especially if you’re not familiar with the source.
After getting a credibility read of mediabiasfactcheck itself (which I’ve done only superficially for myself), it seems to be a potentially useful shortcut. And easy to block if it gets annoying.