Hi Allan,
Does this issue affect other users of the shared mailbox?
If not, does this problem affect other mailboxes in his Outlook?
How about checking in OWA and see if the issue can be reproduced?
Besides, please go to View > Reading Pane, try to clear the checkbox of Mark item as read when selection changes and see if there will be any improvement:

Any updates, please feel free to post back.
Best regards,
Yuki Sun
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