In 0.16, when you create a choose-elem-button and set it to locked, pressing it didn't do anything. In 0.17, it still does not let you change the selected element, but it enters a sort of selected state. Pressing it again makes the selection dissapear and the button returns to normal, like this: (left is normal, right is pressed once)
Now this seems really wierd and unintented, to have a sort of selection state on a locked choose-elem-button. If there is a purpose to this, that's reasonable of course, but I don't see any. So I would advocate to revert it to how it was in 0.16, if it isn't just an oversight.