on focus and mouseover the title attribute is completely removed from the button below, and re-added on blur / mouseout. however once it receives keyboard focus in IE/Edge, the browser seems to "store" the title value even before the focus JS kicks in, and after a short period (500ms?) shows the tooltip (containing the now completely removed title attribute value).

the fix: the browser can certainly wait those 500ms before showing the tooltip, but it should check first just before showing it what the most up-to-date value of that attribute is (and whether it even still exists or not)