Everyone chill??? It's a fictional story. I'm muslim myself and not even offended in the least lmao. The main issue happening here is related to politics, not religion. Kasan pissed about the meat is not "a bad potrayal of muslims", its just him as his own character being a prick, nothing to do with his faith. He's offended as an envoy that the hosting kingdom was indirectly showcasing that they aren't serious to maintain international ties. And before that he took notice that the crown prince wasn't present for dinner.
It kinda falls both ways, its been centuries since the two kingdoms interacted with each other. Kasan was overreacting when he could've just declined the food and explain to people who clearly don't know his culture just as he doesn't know theirs (as evident by his negative judgemental attitude when entering the country), while the crown prince himself should've hosted Kasan's first day in the kingdom instead of prioritising the crown princess selection.
Also kinda the fault of the crown prince, he was lowkey aware the two princess candidates aren't really worthy or that great, yet he took the risk to have them host foreign delegates that he himself have never met or know much of their culture??? Thats a disaster waiting to happen.
Yeah the author is a little innacurate about the meat part but this is a manhwa not a religious class, its a fantasy setting. Some muslims aren't aware themselves of the strict process of slaughtering meat. The author probably didn't do much research into it, but the main message is sent anyways. Take a step back and just enjoy reading free manhwa.