It’s usually considered rude to directly tell a social equal that they’re being rude. Telling someone that they are being weird is perceived as asserting authority over them.
Telling someone that they’re being rude is considered appropriate if they’re someone you’re supposed to have power over. For instance, parents and teachers are allowed to tell children that they’re being rude. Children are generally not supposed to tell adults or other children that they’re being rude (unless they’re babysitting or something.)
Similarly, bosses are sometimes supposed to tell employees to stop being rude — but only if it’s work related. If bosses think that you’re being rude to customers or coworkers, they’re usually supposed to tell you. If bosses think you’re being rude to your spouse, they’re generally supposed to mind their own business.
Telling a social equal that they’re being rude is usually considered rude. It’s seen as asserting inappropriate authority over them. (Eg: you may be seen as inappropriately treating an adult like a child.)
It’s usually considered even ruder to tell someone above you in a hierarchy that they’re being rude. It’s likely to be perceived as implicitly asserting that you’re above them. It’s considered rude to challenge someone’s authority.
Correcting someone’s manners is considered rude in these situations even if you’re right and everyone agrees that you are right. People who say that you shouldn’t have called something rude may completely agree with you that it was rude. They just consider that less significant than your rudeness in pointing out the rudeness.
This gets complicated, because social hierarchies are complicated. And there are sometimes hierarchies that you’re expected to comply with *and* expected not to acknowledge. (Eg: sometimes people want you to obey them but don’t want to think of themselves as the kind of people who have power.)
Another complication is that sometimes people ask you if you think that something was rude. Sometimes they really want your opinion (in which case it’s considered appropriate to give it). But sometimes they want validation (in which case the usual rules about correcting manners usually apply.)
Generally speaking, it depends a lot on your actual relationship with someone. Real interactions between real people are more complicated than any social rules can capture.
This can all be very confusing, and there are no hard and fast rules. It’s very context-dependent. But knowing that it’s a thing can make conflicts easier to understand.
It’s also worth mentioning that it’s not always wrong to be rude. Everyone is rude sometimes, and sometimes it’s absolutely the right thing to do. Sometimes there’s no polite way to stand up for yourself or other people. Sometimes it’s important to do it anyway. (For instance, the Greensboro lunch counter sit ins were extremely rude and extremely important.)
Tl;dr If you’re not supervising someone, it’s usually considered rude to directly tell them that they are being rude. It’s considered even ruder if they are supervising you. Even if you’re absolutely right that they are being rude, it’s usually considered rude to say so directly. It’s more complicated than that in practice, and there are no hard and fast rules. Sometimes it is ok to be rude. Sometimes it’s even necessary to be rude.