A new option is being deployed for Office Click to Run installation for Outlook.

This option is going to set the Reminder pop up on top of all other windows.

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To get it,you need to use Office Click to Run and have the version/build 1805 / 9307.2004 (or later).

And in case you are not aware, you may have to restart Windows (not only Outlook) 3 times before the updated build kicks in.