Messages on server revert to 'unread' after sync

Hi,

I'm using offlineimap to pull down an account just for backup purposes. 
I'm not using a mail reader on the resulting Maildir directory.

I use thunderbird to access the imap server. After running offlineimap 
to pull down emails, numerous messages are marked as 'unread'. Using 
thunderbird, I will mark them again as 'read' but they revert to 
'unread' again after the next run. I run offlineimap once a day.

I'm sure this is probably expected and understood behavior. I'm just 
looking for a 'read-only' copy of my email from IMAP server to a local 
directory. If anyone can help, it would be appreciated.

My offlineimap rc:

[general]
accounts = LocalSync
ui=Quiet
autorefresh=30

[Account LocalSync]
localrepository = LocalMailbob
remoterepository = RemoteIMAPbob

[Repository RemoteIMAPbob]
type = IMAP
remotehost = mail.example.com
remoteuser = [email protected]
remotepass = emailpass
auth_mechanisms = LOGIN
createfolder = true
ssl = yes
sslcacertfile = OS-DEFAULT
sync_deletes = no

[Repository LocalMailbob]
type = Maildir
localfolders = /mnt/ExternalDrive/bob/Maildir
sep = .
createfolder = true
sync_deletes = no


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