Very weird... empty/non-empty button problem/bug

hey… i was just messing around when i had this code

txt.autoSize = "left";
btn.onRelease = function(){
trace("it doesnt work!");
}
btn._width = txt._width;
txt2.autoSize = "left";
btn2.onRelease = function(){
trace("it works!");
}
btn2._width = txt._width;

as you have read, the one does work and the other doesnt… that’s quite weird because there almost the same buttons (not movieclips, buttons)… EXCEPT for this: the one that doesnt work is empty (only the hit tab is filled with a shape) and the other one has the up and hit area filled… i attached i fla to make it more clear

who has a good explanation for this problem(/bug)