read state not propagated

[Marc Roos]

>
>
>>  
>> I have migrated the inbox namespace from mbox to mdbox by doing a 
>> doveadm backup to the new server. Both servers are having CentOS7 + 
>> dovecot-2.2.36-6.el7_8.1.x86_64, both servers have identical 
>> configurations (new server has some different vsz_limits and stats
>> enabled)
>> 
>> Now a user complains that an imap mailbox he opens at the same time 
>> with a colleague does not propagate the 'read' state. Previously if 
>> his colleague read the message, it would show on his mailbox also 
'read'.
>> After the transfer it keeps being unread for him.
>> 
>> Am I correct to assume that he should indeed get an updated to 
'read'? 
>> Where should I look to fix this? What can be the cause of this?
>
>How are they accessing the mailbox? Same credentials?

yes, mostly just 2nd account in apple mail client