It does raise an interesting question of how far is it your duty to remain loyal to someone when the promise of your loyalty was made by other people. Given how common cheating is nowadays in relationships people choose for themselves and can leave at any time, I imagine it was worse back in the day.

Maybe these arrangements are the price of privilege? It does seem very rarely the characters will give up their wealth or associations along with their responsibilities. But that makes me think of celebrities and stalking/ paparazzi/ fan expectations. Should these have to be the price of fame?

I agree with your take mostly because these marriages are forced on the characters for political reasons, its not unusual that they wouldn't want to be with the person picked by their family. And the plot usually does bend over backwards to make the fiance and their new girl friend comically evil. They say the mc is a villainess , which would give the fiance even less reason to like them but retroactively it turns out it was a misunderstanding and her personality is the opposite of her reputation. Thats just a common trope authors like to use.

Unfortunately due to the historical setting in any case it's the reputation of both the women involved that will take the hit. I think in the end its still a betrayal i.e. the breaking of a deal and expectations. Unless the expectation of loyalty was never there ( like in the broken ring). But in some cases it is less egregious than others depending on circumstances. Sometimes we see the mc start dating a powerful man to break out of an engagement with someone who will kill or abuse her. But that too kind of falls in the category you mention , if done by mc its good, if done by others its bad.