Where the div is when you can’t click on it if you trigger the bug going up.
It can be below too, if you go top -> bottom.
Also, I can repro this same behavior with the bottom bar on Instagram. After the first infinity scroll (when it loads more posts) the bottom bar will become inaccessible, depending on if the location bar is visible.
FWIW I use Twitter on mobile not app and realise I’ve seen the same effect when retweeting: the browser was definitely registering my presses about 20 px lower than they were.