When another player in your group leaves, or you leave a group, the name on their nameplate stays the "in party" color.

Its not only color. Whole 'tNameplate.strUnitType' doesn't change, so your options in settings doesn't work if this happens. Maybe some event listener when player joins/leave the group and change their 'tNameplate.strUnitType'

Good to know. Probably want to listen for Group_Left() (for the player) and Group_Remove() (for another player).