A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I really resent that this is necessary, but it's absolutely the fix I needed. Running the second version of that script across all worksheets that used validation wiped out all of the phantom boxes, but the proper list validation drop-down functionality remained intact.
There is no doubt that this is a bug and it certainly seems to be specific to Excel 2016.
For good measure, I added a script that selects a "safe" (unvalidated, unprotected) cell in each worksheet on workbook save events.
Thank you for posting this script. I've been looking for a solution for many weeks now.