First let me say that this may be my database. I saw a couple of hinky things earlier today, but thought it worth a shot looking for the answer here. If this has been addressed before I apologize. I did search and read, and search and read, and se.. well you get it.

I am using ZC 1.54, EP 4.0. It has worked as expected for everything so this threw me. Adding one attribute to all products in spreadsheet, except 6 that would have been the same with no update.
Uploaded/Imported Attributes Basic, reported "Imported with Issues". I recognized the issues because I had used an old spreadsheet to grab my model numbers and a few of those had been deleted, so no biggie. All others say "updated". None were updated.