Emails with blank headers

I have a BT Internet email account configured in Thunderbird using IMAP. I'm experiencing a problem where some emails in the message list have no Correspondent/sender or Subject, and sometimes the displayed time is incorrect. For example, a message-list entry showed 12:55, but when I opened it, the actual email was from 8 September. The email body itself was present and correct. My Inbox contains approximately 7,782 messages and took several days to synchronise. I also have numerous archive folders. I have already: Checked the BT IMAP settings — mail.btinternet.com, port 993, SSL/TLS. Allowed the entire Inbox to finish synchronising. Used Repair Folder on the Inbox. Restarted Thunderbird. Restarted Windows. Changed the message list from Cards View to Table View. The problem persists. Importantly, the same BT mailbox displays correctly in both the BT Mail app and iOS Mail. I also experienced a very similar problem previously in New Outlook: some newly received BT emails appeared as [Unknown] with (No subject). Removing and re-adding the BT IMAP account and repairing/resetting New Outlook did not resolve it. Thunderbird is therefore the second desktop IMAP client in which I've experienced this type of problem, while BT's own mail app and iOS Mail display the messages correctly. I don't want to create a new Thunderbird profile because the existing profile contains thousands of emails and took several days to synchronise. Could this be an IMAP synchronisation/message-header/indexing issue between Thunderbird and BT Mail? I can provide screenshots and any Thunderbird diagnostic information you need.