That's why I never buy strategy games at launch. They take a few patches to fix the bugs, balance the game and improve the AI.
I feel like I've had that war scenario happen to me as well in many of the previous civ games. It seems it depends on map layout. Being denounced out of the blue, is also pretty normal. What I'm finding odd is the statements that they make which aren't denouncements, but are downright perplexing. I am finding the standard "Prince" difficulty (4) much easier than past games. I am a huge fan of the series, back to '94 with Civ 1. So far, I like the new direction with VI.
If you liked civ 5 biggie you'll like this, more of the same with some improvements. Playing this though after playing endless legends and stellaris, it's weird having no in game plots or objectives outside win the game.
I can't seem to get into Endless Legends. It's clearly a solid, polished Civ clone and I really like the new ideas. I think it's the fantasy elements that turn me away whereas with Civilization it's "historical". I can be Ghandi and be a warmonger. I can be Ghengis Khan and be diplomatic.
Free on Epic Game Store: https://www.epicgames.com/store/en-US/product/sid-meiers-civilization-vi/home
I guess this game isn't very popular around here. But I bought the Anthology edition yesterday (85% Steam sale). Played for about good two hours today. Great fun!