When ask to stop trading or when I ask others to stop trading, some times open border are canceled, other times resource exchange stopped. kinda confusing.

It cancels all ongoing deals with between the two, puts the target into "refuses to talk for a bit". It also gives a -1 demerit for the person who bribes the embargo (not sure if the AI gets it for making you stoptrade, but you do if you bribe an embargo). The person who stops trading also gets a -1.

You can avoid this by canceling borders ahead of time.