Message with new message icon even if they are read

Hi All,

Some users complain that some messages in their mailbox have the new message icon and message have marked as read.

I used MFCMapi.exe and verified that the mailbox that reports this problem, the message has a property call PR_ICON_INDEX and the other doesn’t have this property.
In the post https://learn.microsoft.com/en-us/office/client-developer/outlook/mapi/pidtagiconindex-canonical-property we can see the value of the PR_ICON_INDEX properties and the message with this issues have the value of 0x00000000 and this value does exist in the article.
If I delete this property, the icon disappears.

My question is:

Why this mailbox have different behavior?

Why the property PR_ICON_INDEX of the message have the an unknown value of 0x00000000?

We have Exchange Server 2019 CU14 Mar24SU on-premises, Outlook 2016 and use OWA to access the mailbox.