read state not propagated

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?