I had this same exact issue, and that’s what prompted me to try removing the mail app, rebooted, and reinstalled Mail from the App Store. After adding my accounts back, ALL of my emails showed No Sender/No Subject. And that’s for all 6 of my email accounts (iCloud, 3 Gmail accounts, a Yahoo account, & an IMAP account). Sooooo, I filed a radar and it gets marked with the “more than 10 people have reported this” tag at the top. Meanwhile I’m stuck having to use Spark because I’m literally unable to search any of my folders for a person’s name. Can’t search for anything that could be in the subject line. There’s no way I can really use the Mail app at all. I’d use the Outlook app but I use that just for work email. Nice keeping that separate.