In an effort to determine why mail has gone missing, it may be helpful to search the maillog for pop deletions.
One way to do this is with the following command, making sure to replace email@example.com with your email address:
grep firstname.lastname@example.org /var/log/maillog* | grep -v "del=0" | grep del
The output would look something like this:
/var/log/maillog-20210423:May 23 01:04:19 user dovecot: pop3(email@example.com)<1682746><yE8VNpPCzkS+cdf5>: Disconnected: Logged out top=0/0, retr=0/0, del=7/232, size=145286042, bytes=88/5960