It's possible that there was a problem when the pdf file was renamed. Note that Zotero's attachment titles and filenames can be independent. You see both of them in the right-hand attachment box, when the focus is on your pdf. The title is in the top row, the filename is in the row that starts with "Filename:". If the title and the filename have been set up to match, but do not match, this could be due to a renaming error.

Press the "Locate..." button and check if you find a pdf file in the 'MC9LAEVG' folder. If so, select it and click "Open". Then try to open the file in Zotero.

I sometimes experience similar issues when importing many items at once through the browser connector. The issue seems to be that the file is renamed on the file system, without updating Zotero's data base. (It could also be the other way around.)

As in your case, the issues I'm experiencing occur with stored files. There's a separate folder for each pdf file, 'MC9LAEVG' in your example. So the issue has nothing to do with a naming conflict. In some cases, a related issue might occur when the pdf file is opened in some viewer, which prevents a renaming operation on the file system. But this wouldn't explain the issues when importing new items from a journal website. So there seems to be some timing problem that occurs only occasionally. Network speed or system load might be factors that are relevant. Luckily, this is not critical, since the file is already in the proper folder and using the "Locate..." dialog solves the problem.

A related issue on github is: https://github.com/jlegewie/zotfile/issues/463. There, it's reported that the issue appears regardless of whether Zotero or Zotfile renames the file. I don't know whether the issue can actually be reproduced with Zotfile disabled. I'm using Zotfile to rename files, and the issue doesn't occur frequently. Properly identifying the issue might require heavy testing or an in-depth look at the code.