aetfor, By any chance are your Local Folders using maildir storeage format and not the default mbox? I have my Local Folders set to maildir for some reason I don't remember. Also, I can see one of the 6 messages copied and the other 5 are empty/blank. Looking at the maildir storage files, they are all mostly empty except for the one I can see.
Bug 1781792 Comment 12 Edit History
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aetfor, By any chance are your Local Folders using maildir storeage format and not the default mbox? I have my Local Folders set to maildir for some reason I don't remember. Also, I can see one of the 6 messages copied and the other 5 are empty/blank. Looking at the maildir storage files, they are all mostly empty except for the one I can see. I also see this each time I cause the filter to run using multiple messages. So it's not just a startup problem.