Thanks. That was the issue in my case.

The default configuration is: * Mark as read after selection changes

Which in my opinion is really weird. I've been using email for 20 years and no other client I've used works this way. It's so inefficient since you need to click into a previously read email to mark a different email as read.

But, I did switch things to mark it as read when chosen which fixes it.

Oh web outlook ugh. People where I work hate Outlook web. It's not intuitive. Shared mailboxes in web Outlook are a pain they dont show on the left sidebar they need to be opened in a separate tab. Terrible for efficiency. Except "out of office" which for shared mailboxes it can't be disabled in non-web Outlook you have to do it in web.

My day and job are based on Microsoft weirdness.

One big annoying thing is that when you mark an email as unread in Outlook on mobile, the default is to mark the entire email chain as unread. So if you have a long email chain, all of sudden there will be like 20-30 unread messages in your inbox. It should have the ability to mark unread from here or something like that.