Welcome to the Airtable community!
Airtable automation triggers are sensitive. For example, with a text field, each key press is considered an update, and an automation for “on update” would fire for each key press. In the case of an attachment, attachments sometimes go through a multi-stage upload process as the attachment is uploaded, moved to a different server, and thumbnails are created. This might be causing your problem. You can try adding a delay to your automation to see if that helps. I also recommend telling support about your experience.