I recently enabled auto_complete and I just remembered why I disable it in the first place: it’s a big buggy. To easily replicate the issue, you have to increase auto_complete_delay to a larger value (e.g. 500) and then, while you’re typing, hit arrow keys (doesn’t matter which direction) just before that timing.
You can see a recording of this here (you’ll notice that the popup shows up when the cursor is on the next/prev line):
(the video is recorded on a portable ST with no plugin or customisation)