I am getting the same peculiar behavior with that dropdown box as you are and using NVDA. What I have noticed that ALT+K, followed by the first letter of the item you want to search, works to set it. You do have to do this twice if you want to look in Appointments as there is an All category that will come with the first letter A. Here are the dropdown box options:
Any type of Outlook item
Appointments and Meetings
Messages [default at initial opening]
Another thing I'm noticing, and it's not 100% consistent, is sometimes after you do an ALT+K, letter combination the Advanced Find box disappears from focus and visual view, but it is not actually closed. You need to bring it back up via ALT+TAB.
Both of these things seem to be Outlook bugs and you may want to get Microsoft Accessibility involved and demo them for them so they can see it actually happening.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
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