This is fixed.
I've actually run into this problem in the past with an internal autocomplete that would show "No Records." on complex expressions and then wipe out your text if you clicked it. We were using two versions of an old unmaintained library. One was used on any page that had tag autocomplete and the other was used a handful of other places, like the community picker.
For whatever reason, the non-tag version has a feature where it displays a selectable "No Records." item when there are no matches which actually inserts the text "No Records." on select. The reason it shows "undefined" is because we pass a rebuilt array of objects into the function and display
row.name but when there are no rows,
row == "No Records." and
"No Records.".name == undefined. There's a whole can of worms with the options I used trying to fix this, but I won't go into that because...
I deleted that version of the library and am instead using the one bundled with tag autocomplete everywhere on the site. That version actually solves this problem and has been maintained internally over the years, so it's best to just get everything in one place.