[DNSB]

I think the issue is the OP is running Autoadd when a download is in progress to the Autoadd folder. During the download, the file is in two parts. A dummy file with the filename and the in-progress download with .part appended to the file name. i.e. ebook.mobi and ebook.mobi.part. calibre should not be adding the .part file since it is not a supported ebook type unless someone has ticked the Automatically add files of all types (not just ebooks) under the Autoadding options but the original filename will be seen as addable and since Autoadd deletes the file after adding, can we say ooops?

BetterRed's suggestion to add the .part extension to the exclude rules is not likely to help in this case.

Sorry but to me, this is a PEBKAC error.