That's not a bug its an annoyance. While you have the equipment window up you have to click on the portrait of the individual you want to equip the item too or it will go to the main always. Like setting focus you physically have to click on each character while they are all displayed.
If you looking for Individuality of equipment assignment, I've found a good way to do it.
1. in Inventory pass the item you want to equip to the character.
2. Close inventory window.
3. click on your party portrait that item is on.
4. hit the "N" key to display that single characters gear load out screen and right click on that item to equip it .
