Have you looked at the limits yahoo place on IMAP? My understanding is you may have exceeded the number of email they are prepared to actively synchronize.

See https://www.reddit.com/r/Thunderbird/comments/v4ydu0/10000_email_limit_via_imap_in_yahoo_email/

Thank you for your reply. I use export.imap.mail.yahoo.com to sync my emails, which has a limit of 100k emails. I currently have about 97k emails, so I am within the limit.

You possibly selected "Delete all but the most recent xxxx messages", where 2000 is the default value. You can find this in your account settings, under Synchronization & Storage. Re-syncing 97K messages might appear as stalling.

Thank you dillenger for your answer. Under Disk Space in Synchronisation & Storage I have Synchronise all messages locally regardless of age, Don't delete any messages and Always keep starred messages selected. Besides, if this was the case, Thunderbird would also be deleting messages from the server too, which it does not do.