New Outlook randomly not showing certain emails in inbox?

We’ve encountered a fairly rare but reoccurring issue where an email is successfully delivered to a user’s inbox but does not appear visibly in Outlook. Despite not being immediately visible, the email I am able to locate the email using search, which also confirms that the email is in the inbox folder. The user typically becomes aware of this missed email because the email appears on their iPhone.

A few notes:

The user has inbox rules set, but none that would affect the email in question.

There are no special views or filters applied in Outlook.

Focused Inbox is enabled, but the email does not appear in either the “Focused” or “Other” tabs.

The user’s inbox is not full, nor is it close to capacity.

The user is on the new version of Outlook; however, I’ve observed the same issue with the classic version.

Message trace confirms the email was successfully delivered to the inbox.

Microsoft support has stated that there is nothing wrong with the user’s mailbox.

I have tried search for this online, but a lot of the stuff I find mention the same things and typically don’t have a resolution.

I’m really hoping someone here has encountered this, and (hopefully) has a fix. I honestly feel like it’s one of those unnoticeable bugs or something that just gets typically shrugged off.