This is one hell of a bug. Kongo asked me for friendship, I agreed. Two turns later his units are attacking mine even though our friendship is active. 
Have a save file...

- He has an entire army in my lands, but happily pays me 40 gold for open borders (he initiated the trade).
- During the war asked me to allow him to establish an embassy...
- I capture the first city from him, he again asks me for an embassy.
- His status is friendly/declared friend and he looks like we're allied during trade deals.
- Didn't even see an option to request peace, just the make a trade deal button instead
Have a save file...