I haven't seen this as a bug report so hopefully I'm not duplicating anyone's topic.. but here it goes.
When I'm ranging with my msb on rapid, and I switch to a melee weapon (scimitar, or even just my hands) it will switch it automatically to block. This also works vice versa, putting nothing on and putting it on block, then equipping my bow it puts it on rapid.
Example when this would be annoying: Trying to pk with bow with a melee weapon as a combo but you don't want defence XP.