I see this when the messages are encrypted or digitally signed. Both things are technically attachments, but when you open the mail you see neither

Thanks, Matt! I don't see that encryption could be involved here. I can additionally report that the false attachment icon associated with an unread message disappears when it is read.

There are more types of attachments than those two, they are the ones I see commonly. I also see it in mailing lists where listservers build the email out of attachments. Attachments that are meant to be viewed inline, but attachments none the less.

Try installing this add-on https://addons.thunderbird.net/en-US/thunderbird/addon/show-all-body-parts/

Then change the view menu > message body as entry to all body parts. It might be enlightening.

Matt, I was prepared to try your suggestion but found to my surprise that the icon is now behaving normally! Looks like it unflumoxed itself somehow. I made no changes nor was I aware of any update that would account for this. Thanks for your attention!

My problem is similar: the attachment icon does not always appear in the list view. The attached screenshot shows a highlighted line with attachment shown, and the top line in that list is a resend of that same email using "Edit message as new". It has the same attachment, but the attachment icon is not visible. However, if I highlight that top line, the attachment icon shows up. My platform is Ubuntu bionic beaver.

Thanks,

Norm