Read/unread message syncing

Read/unread message syncing

Not sure if this is a Thunderbird or Zoho issue. 

A client of mine mentioned he was having some IMAP sync issues with Zoho and Thunderbird.  I use the same combination and hadn't noticed an issue until today. 

I have a message that I read on on my iPhone.  It is marked as read on my iPhone and in Zoho, but still marked as unread in Thunderbird - more than an hour after reading it in my iPhone. 

Deleted messages sync fine between the three locations.

Any ideas?

Scott