Yes this has been an issue for quite some time now, it happens most of the time when attacking npcs larger than 1x1 tiles, the only workaround seems to be the EntityDestination method
EntityDestination npcDest = new EntityDestination(getBot(), npc);
which you can use to hover and attack with your mouse. Personally I hover it then call interact if the mouse hovers over it, and for me it has been fine.
However, don't worry, as the devs are working on a fix for issues like this.
http://osbot.org/forum/topic/68748-