I noticed this as well. Two enemy parties that were right next to another and clearly within reinforcement distance of eachother. They also attacked me together. Still i only had to fight each party separately.
As the title says sometimes the moment you start talking to an enemy party the mouse cursor vanishes. When this happens i am unable to select anything.
The only way to fix this is to restart the game.
This only started happening after upgrading to 1.5.8.
This happens really annoyingly often...