Are you accessing the account through any mail apps or clients? If so, these could be retroactively moving emails off of our servers. I would remove and re-add the account if so. To recover missing emails, you can also submit a restore form online.
I am having the same problem. I think this just started today. It certainly started within the last couple days. I access my yahoo email on both my laptop (via IMAP in Outlook) and my iPhone. The settings are to keep all emails. The emails are not in the Trash folder. We are talking about years of emails so there is zero chance I accidently deleted them.
I access another Yahoo email account in the same manner and am not having this issue.
Any ideas on how to restore the emails?
You can go here to submit a restore request through our automated system. Be sure to check the Trash folder when looking for restored emails. If you're accessing it via IMAP in Outlook, you may want to remove and re-add the account once the emails are restored to keep this from happening in the future.